The Boston Celtics have the second-best defense in the league through the first two weeks. We still aren’t at a point where we can determine whether this phenomenon is for real or simply just a really good stretch where they only went up against one team that ranks in the top 10 offensively. Still, Boston has been downright impressive throughout the early part of this year due in large part to the team’s switch to longer and more versatile defenders. Let’s take a look at the numbers.

The Stats

The Celtics are currently second in the league in half-court points allowed in the league via Cleaning the Glass. This means that per 100 possessions in the half-court, the Celtics are allowing the second-fewest points. Furthermore, Boston ranks 3rd in the league at defending transition possessions off of live dribbles. So when teams have tried to push the ball after a Celtics miss, they’ve been one of the best at the league at getting back and stopping the opposing offense. Boston is 17th in opponent three-point attempts, and 2nd in opponent three-point percentage. To expand on that stat, via Cleaning the Glass, the Celtics are 4th in opponent three-point percentage from the corner, and 2nd in non-corner threes. In other words, Boston has been elite in defending the three-point shot in all situations despite the fact that they’re currently only 11th in opponent corner three frequency.

Overall, this picture paints the team has an elite defensive unit that is good enough to win team games, the next question is how does a team that lost Avery Bradley and Jae Crowder somehow look better on the defensive end?

Length, Length, and more Length

Last season’s Celtics defenders got a lot of credit that was well deserved, but as a unit they struggled. There were a lot of different reasons why, but the main ones were a lack of rebounding, no legitimate big, and trying to overcompensate for Isaiah Thomas. The Celtics front office noticed this and went out in the offseason looking to replace the small and scrappy roster with longer and more versatile defenders. So out went Amir Johnson and Avery Bradley, and in came Jaylen Brown and Aron Baynes. The tradeoff on it’s face doesn’t seem like a clear upgrade, but viewed as a whole, the Celtics have gone through a dramatic shift, and it’s not hard to see when you watch it live:

With all that added length, the Celtics have been able to do a couple of things. Take this play for example:

The Heat are one of the best teams in the league at driving to the basket. Because Boston is playing two long wings against Josh Richardson and Dion Waiters, the rest of the team is able to stay at home and not overreact to penetration, which limits drive-and-kick opportunities. Irving will get the credit for the steal here, but it was Brown’s ability to use his length and stay with Waiters even when he did get a step on him that caused that turnover.

Along with the ability to stay home, Boston’s newfound length has led to the team becoming one of the better turnover generators in the league. So far the Celtics are 9th in the league in forcing turnovers and 3rd in deflections per game. Guys like Irving, Rozier, and Smart have been hyper-aggressive in the passing lane and all have steal percentages over 30. The surrounding length has made life difficult for opposing teams and has made Boston’s defense even more smothering.

When they’re not forcing turnovers, Boston has been great at simply communicating through pick and rolls, switching when needed, and trailing right behind their man as they try to run through their sets.

Irving will be the only one credited on that play with a great block, but the activity before that is what won the possession for the Celtics’ defense. This play was set up for Brogdon after coming off staggered screens from Middleton and Antetokounmpo. Theis sniffs it out and immediately helps over when it appears Brown was a step behind, while Horford and Irving essentially face guard Middleton to make the pass impossible. Then, Theis hustles back up to defend the Maker shot but doesn’t charge at him and take himself out of the play in case of a pump fake. Unsatisfied, Maker then passes back to Antetokounmpo who gets it to Middleton, which in theory is a mismatch. But Irving isn’t moved by Middleton’s bump to create space and is able to get a block. That blend of strong individual defense with a mix of more versatile athletes has helped in turning this average defensive unit to one that drives opposing coaches crazy.

The X-Factor

Besides the solid team defense and added length, one of the best surprises has been Al Horford DPOY-like performances. The ancillary parts that have helped put him in position to have this type of impact have been Aron Baynes and (surprise) the team’s added size on the perimeter. The Baynes addition has allowed the Celtics to limit Horford’s possessions against opposing bigs, permitting him to roam on the perimeter. Besides the added advantage of not getting beaten up down low, Horford has been an absolute menace on the defensive end, flying from end to end aggressively calling out “ICE” and constantly communicating to the other perimeter defenders in a quarterback-like role. The added ability to attack the boards instead of boxing out has lead to increased rebounding numbers (23.3 Dreb%) and more Horford pushes in transition.

Horford has a stifling 91.3 defensive rating, which ranks 2nd amongst centers who have played 20 or more minutes. The latter stat is nothing to sneeze at—during this stretch he has been tasked with defending Giannis Antetokounmpo, Kristaps Porzingis, Kevin Love, Joel Embiid, LeBron James, and Ben Simmons. Here’s how they’ve fared when guarded by Horford:

That’s 31% shooting, folks. Needless to say Horford has been an absolute terror on the defensive end, and for a few of these matchups, he has been the difference-maker.

During the first matchup against the Bucks, the Celtics tried to throw their wings at Antetokounmpo and failed. Brown fouled out, Tatum couldn’t contain him, and Baynes got put on multiple posters. The second time around, the Celtics decided they weren’t going to try and use wings that might be quicker, they’ll just put Al Horford on him and do their best to pack the paint. When Antetokounmpo tried to attack, he saw this:

Horford was too strong to gain much position inside, and he was long enough to give a decent contest when Antetokounmpo did settle for mid-range shots. The end result was a frustrating night that only looked good on paper after back-to-back threes during garbage time.

Horford’s ability to defend these in-between stars like Simmons, Antetokounmpo, and Porzingis gives Boston a secret weapon that can be deployed in what could be first- and second-round matchups for the next three years. A big part of what allows him to defend those players is having surrounding defenders that can hold their own in their individual matchups combined with a legitimate big man who can deter drives to the paint and do the dirty work of boxing out.

What should we expect in the future?

As good as the Celtics have been defensively, they do still have some room to grow. Communication on switches can be a little choppy at times, with guys not knowing who should stay on the man who set the screen and who should chase the man for whom the action was planned. They’ve also been caught sleeping a couple of times on side pick-and-roll actions where the wing gets up court and quickly passes the ball to the guard coming from the corner. The guard then stops right behind the screener and has an open look at the three if his defender tries to go around the pick instead of trailing. Here’s an example:

The good news is little details like that can improve as younger players continue to get playing time and start learning not just each other’s tendencies but also the tendencies of opponents.

Boston still has a long way to go before the rest of the league starts catching on to their new and improved defense. It’s rare to see a team with an average age of 24 have such young players contribute so convincingly on the defensive end. It’s only two weeks, but hey, a quick look around the league tells you things could be much worse considering the circumstances. It might not be time to make any conclusive claims quite yet, but the numbers do suggest that this team is on track to being one of the elite defensive units in the league.