Chris Forsberg, ESPN (http://espn.go.com/nba/story/_/id/14799814/how-boston-celtics-built-top-3-defense)

For 16 dribbles — an NBA eternity — Denver Nuggets rookie point guard Emmanuel Mudiay tried to figure out a way inside the invisible force field that the Boston Celtics and their switch-happy defense had constructed at the 3-point arc. Every time Mudiay tried to get into a pick-and-roll action, Boston simply switched defenders, unfazed by the fact that, at times, it left center Amir Johnson guarding Mudiay or undersized Avery Bradley covering a big like Danilo Gallinari.

Boston’s switches eventually led Bradley, who former teammate Rajon Rondo still calls the best on-ball defender in the league, back to Mudiay. As the shot clock reached single digits, the rookie — with no other option — tried to drive, and Bradley, as he has done countless times before, shuffled up to impede his path and then utilized a well-timed right-handed reach to strip the ball away.

“I’m jealous of the defenders they have,” Nuggets coach Mike Malone said of Boston’s perimeter defense, even before the Celtics harassed his Nuggets into 20 turnovers while generating 15 steals during that late January matchup. “They have a team of grit and toughness.”

Grit, toughness and, most important, versatility. It’s the backbone of a Boston defense that somewhat improbably owns the third-best defensive rating in the league while allowing 99.6 points per 100 possessions.

As part of the team’s on-the-fly rebuild, Celtics president of basketball operations Danny Ainge has armed coach Brad Stevens with a roster of young, defensive-minded players with the sort of versatility that allows Boston to switch on the perimeter — limiting the amount of dribble penetration and masking the Celtics’ lack of a true rim protector.

Boston’s defense is a primary reason the team will tip off the second half of the season on Friday night against the Utah Jazz (ESPN, 10:30 p.m. ET) with a 32-23 record and sitting as the third seed in the Eastern Conference.

And it’s that defense that might just dictate exactly how successful the Celtics can be in the postseason. Which is why, when the Celtics reconvened out west earlier this week, Stevens put his team through a film session heavy on getting Boston back to a defense-first team despite a recent uptick in offensive production that helped the Celtics win 10 of 12 games before the All-Star break.

“We’re an 80-percent-of-the-time [defensive] team, and maybe that’s good. Maybe that’s as good as anybody else may be,” Stevens said. “I don’t know. I’m not watching everybody else play. But we’ve got to get it up higher to compete against the best. We’re not gonna win against the Clippers giving up 134 points anymore. I mean, that’s just a unique night where you throw enough in to win. We’ve got to get back to guarding and being who we are at our best level.”

“Can we get a little bit better?”

Two years ago, on the eve of his first NBA trade deadline in 2014, Stevens huddled his team in Phoenix and issued a challenge for the second half of the season: Generate a couple of more stops per game and see what happens. Stevens was under no delusion that the Celtics, then 17 games under .500, were going to make any sort of second-half charge. But ranking 14th in the NBA in defensive rating at that point, Stevens wondered if his team might be able to claw its way toward the top 10 over the final 28 games of the season.

It didn’t happen. Boston stumbled to the finish line that season, losing 22 of its final 28 games while ranking 24th in defensive rating in that span. The lone upside to Boston’s late-season struggles was that the Celtics landed the No. 6 pick in the 2014 draft and snagged Oklahoma State guard Marcus Smart, who came as advertised with NBA-ready defense.

When Bradley, one of Boston’s only remaining players from the Big Three era, met with reporters at his Boston-area basketball camp in August 2014, he gushed about the Celtics’ defensive potential and offered a declaration similar to what Stevens had challenged his team six months earlier.

“I feel like we have a chance to be a top-10 defensive team in the NBA this year,” said Bradley, whose proclamation was met with laughter by most observers. While Bradley is admittedly one of the more optimistic players in the league, he truly believed it was an obtainable goal if the team bought into Stevens’ philosophies.

But over the first 24 games of the 2014-15 season, the Celtics owned a 10-14 record and ranked 15th in the NBA with a defensive rating of 103.1. This wasn’t exactly the progress Stevens or Bradley had hoped for, and it was clear that Boston needed a bigger overhaul to get where it wanted to go.

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“Defense is our identity”

Perched on top of the scorer’s table inside TD Garden with an overflow throng of media surrounding him during his first trip back to Boston in a visitor’s uniform last January, Rondo was asked about his seemingly renewed defensive vigor in the aftermath of the swap that delivered him to the Dallas Mavericks.

“I haven’t played defense in a couple years,” Rondo said. “Here they expect me to play defense.”

It was Rondo being Rondo. In one breath he admitted what had been painfully obvious — that his defensive effort had waned and, without Kevin Garnett to cover for him on the back line, Rondo’s defensive shortcomings were highlighted at the end of his Boston tenure.

But make no mistake, defense is a requirement in Boston. From the first day he was hired in July 2013, Stevens attempted to instill what he calls a defensive DNA. And the No. 1 priority in establishing that is having every rotation player commit 100 percent to that side of the ball.

It’s probably no coincidence that Boston’s defensive turnaround — from lingering in the middle of the NBA pack to now rubbing elbows with the defensive elite, such as the Golden State Warriors and San Antonio Spurs — started the exact moment that the Celtics swapped out Rondo for a package that included defensive bulldog Jae Crowder.

“I think our mindset, night in and night out, is trying to be a defensive-DNA-type team,” Crowder said. “We talk about it and all guys are trying to buy into it and you’ve got to have that mindset. I think once you have that mindset and try to put it into play, it’ll come to life for you as a group.”

Over a 36-game span starting in early February 2015, the Celtics owned a defensive rating of 100.6 (eighth best in the NBA over that period) while posting a 24-12 record and surging to the No. 7 seed in the Eastern Conference. As part of that run, the Celtics made two more deals at the trade deadline, most notably acquiring offensive spark plug Isaiah Thomas from the Phoenix Suns.

Thomas played on some poor defensive teams early in his career and was regarded as a liability because of his size. Most assumed the Celtics were willing to live with those, ahem, shortcomings because of the jolt 5-foot-9 Thomas could provide offensively. Asked if Stevens stressed the importance of defense to Thomas upon his arrival, Thomas smiled and offered, “No, he talked to me about scoring that ball.”

Turning more serious, Thomas added, “[Defense is] our identity. That’s definitely our identity. We want to be known for that. [Stevens] just says, whatever your job is, do it at the best of your ability, but know we have to hold our hats on the defensive end.”

Thomas has acknowledged that he might be Boston’s weak defensive link, but he has worked hard to improve on that side of the ball. With this team, he really doesn’t have a choice. The Celtics have done a good job of putting Thomas in position to thrive, allowing players like Bradley and Smart to defend top scoring guards, while Thomas can roam a bit while checking a spot-up shooter, even if he’s giving up more size.

The results? The league’s player-tracking data shows that Thomas is limiting opponents to 26.1 percent shooting on 3-point attempts, which is 9.4 percent below those players’ season average. Sure, Thomas struggles to defend closer to the basket, but he’s thriving in his primary responsibility.

The Celtics have been a mainstay at the top of the league in defensive rating throughout the 2015-16 season with only occasional lapses in intensity.

“We’re just got a group of guys that can defend at a high level,” Thomas said. “A group of guys that defend on a string and help each other out. That’s what defense is about. Half the battle is talking and communicating, and the other half is just having the pride to go out there and stop the opponent. So we have a lot of guys that take pride in that, and it makes it easier for the rest of the guys.”

“We play hard, and a lot of guys don’t like it”

For the first time in nearly two decades, the Celtics entered this past summer with available cap space to chase free agents. Knowing that the team was unlikely to hook one of the few available big fish, the Celtics instead zeroed in on free-agent big man Johnson. While undersized for a center, Johnson was one of the best defensive bigs available and instantly became Boston’s best rim protector when he inked a two-year, $24 million deal.

After mixing and matching a bit at the start of the season, the Celtics settled on a starting lineup that has featured Johnson and rebound magnet Jared Sullinger up front with Thomas, Bradley and Crowder on the perimeter. The group hasn’t exactly been dominant defensively — owning a defensive rating of 102.3 this season — but their consistent offensive output (105.1 offensive rating) has made it an effective first unit.

And then Boston unleashes its secret weapon: a reserve group that might actually crank up the defensive intensity, particularly while mixing Bradley heavily into backup units that feature Smart, Evan Turner, Kelly Olynyk and Jonas Jerebko.

Just check out the list of Boston’s individual leaders in defensive rating this season: Olynyk (96.5), Jerebko (97.5), Smart (98.2) and Turner (98.6) are tops on the team among regulars. How exactly is that possible considering that guys like Olynyk and Turner have rarely been regarded as strong individual defenders during their careers?

It boils down to that versatility. The Celtics feel comfortable switching most pick-and-rolls, and when Boston defends the 3-point line — something it’s been inconsistent with at times — it can be a very tough team to score on when playing in a half-court set.

The other key for Boston: creating havoc. When the Celtics are engaged — getting into the bodies of opponents and clogging passing lanes with active hands — they create turnovers at a high rate. Boston had a four-game stretch in late January, including that win against the Nuggets, in which it forced at least 20 turnovers per game. For the season, the Celtics rank fourth in the league in opponent turnover ratio (16.5 per 100 possessions) this season.

And the Celtics take pride in being the bullies.

“We play hard,” Crowder said. “Guys don’t like that in this league. Guys want an easy, flowing game. But we play hard, and a lot of guys don’t like it.”

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“Defense has consistently given us a chance”

Ask Bradley if he’s surprised that the Celtics have elevated to one of the top defenses in the league and he doesn’t hesitate with his response.

“Of course I’m going to say ‘No,'” he said.

But here’s the real surprise: Bradley thinks Boston can still take its defense to another level.

“I feel like the sky is limit if we continue helping each other and talking,” Bradley said. “I feel like we still don’t talk as much as we could, and look how good we are some nights.”

Stevens, a big four-factors stats guy, thinks the Celtics can improve their defensive rebounding (25th in league with a defensive rebound rate of 74.6 percent) and wants his team to limit its fouling (29th in the league with an opponent free throw attempt rate of 0.316). Offensive rebounds and putting opponents at the charity stripe have negated some of Boston’s defensive efforts this season. And even as the Celtics have started to play better offensively lately, Stevens has often reminded his team what butters its bread.

“If you are playing well on the defensive end, you are going to give yourself a shot, no matter what,” Olynyk said. “That’s what we need to do every night, gives ourselves a shot at the end. If we are playing good offense, we are going to beat teams. If we are not, then we need to rely on our defense to give us a shot to win.”

When the Celtics are fully engaged defensively, they are a fun team to watch. Smart, a noted antagonizer (ask Russell Westbrook), had a sequence against the Brooklyn Nets in which he laid out to snatch a loose ball when Donald Sloan tried to walk the dog. When the Celtics were struggling to slow New York Knicksrookie Kristaps Porzingis, Stevens dispatched Smart despite nearly a foot difference in height.

Crowder has saved some of his finest defensive efforts for when the league’s top small forwards are across from him, hounding Carmelo Anthony or chasing LeBron James (even while battling a high ankle sprain during Boston’s recent win in Cleveland).

Los Angeles Clippers coach Doc Rivers, who coached Bradley while with Boston, said of the Celtics during a recent visit: “They have three Averys — with Crowder and Smart. I know there’s not a better 1-2 combination in the NBA, defensively, and there may not be a better 1-2-3 combination, defensively. They’re tough.”

Boston’s offense is too inconsistent for the team to lean on. Stevens doesn’t expect many 139-134 overtime victories like the one Boston posted over the Clippers right before the All-Star break. No, if these Celtics are going to consistently beat good teams — something they’ll have to do to take a step forward this postseason — they’re going to have to consistently bring their best defensive effort.

“Our defense has consistently given us a chance,” Stevens said. “That has to be our calling card.”