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What caused the Celtics dramatic three-point defense drop-off?

Boston fell from 2nd in the NBA to 22nd in opponent’s three-point percentage, ending a 13-year run of top-7 finishes in the category

NBA: Brooklyn Nets at Boston Celtics Gregory Fisher-USA TODAY Sports

Three-point defense in the NBA has been a mystery box for years. As the three-pointer has risen in importance, one would think the importance of defending the arc would rise on a similar trajectory. It’s not uncommon for teams to rise or drop as many as 15-20 places from season to season in terms of opponent’s three-point percentage. While teams have different approaches to how they defend three-point shots, usually based on limiting attempts or certain types of attempts, no team has figured out a way to consistently make opponents miss from deep.

Except for the Boston Celtics. Well…until the 2020-21 season that is.

At the start of the 2018-19 season, CelticsBlog published an article that showed how Boston had cracked the code on defending the three. From 2008, when Doc Rivers and Tom Thibodeau built a championship defense, all the way through 2020, the Celtics didn’t finish lower than 7th in the NBA in opponents three-point percentage.

Rivers and Thibodeau put in a strategy of running hard at shooters and challenging them. Not just the best shooters, but every shooter at the arc. Under Brad Stevens, Boston amped this up by not only requiring that their defenders close hard to a shooter, but asking them to jump to contest shots. Both approaches resulted in fouling three-point shooters at a slightly higher rate than other teams, but the tradeoff of forcing misses was more than worth it.

Where others had found some success in limiting three-point attempts, by running their opponents off of shots, the Celtics were content to let you shoot them. You just weren’t going to get very many good looks.

How big of an anomaly was this 13-year year of three-point defense dominance? During the same period, no team had put together more than a three-year run of top-7 finishes in terms of percentage. Some were, and still are, regulars atop the league in limiting attempts, but none have been as effective as Boston at limiting makes.

Looking at the 2019-20 season vs the 2020-21 season, we can see some trends that support this variance. The Utah Jazz have made it a focus under Quin Snyder to limit opponent three-point attempts. The Jazz finished 3rd in both seasons in attempts allowed per game. This season, as Utah put together the NBA’s best record, they held opponents to just 34.1% from behind the arc, good for second-best in the league.

In 2019-20? Despite largely the same personnel and also limiting attempts, opponents shot 35.3% from three, which was 12th in the NBA.

How about a more extreme example? The Minnesota Timberwolves were a terrible defense this season. They were last in the league, allowing opponents to hit 39.2% from downtown. The year before, with a similarly terrible defense, the Wolves were 20th in the NBA allowing 36.3%.

Want to go the other way? The New York Knicks, under Tom Thibodeau, made a Boston-like leap. The Knicks allowed roughly the same number of three-point attempts per game in 2020-21 as they did in 2019-20. In 2020, New York finished 28th in the NBA, allowing opponents to hit 38.1% of their deep balls.

In 2021? New York was atop the NBA at allowing opponents to make just 33.7% of their three-pointers.

There’s clearly something at work here that Thibodeau brought with him to New York. Even Doc Rivers, who had the Philadelphia 76ers atop the Eastern Conference in his first season, has carried over some three-point defense philosophy. The Sixers allowed more attempts this year, but bumped up slightly from 13th in opponent’s 3P% in 2019-20 to 9th this season.

Boston Celtics v Philadelphia 76ers Photo by Tim Nwachukwu/Getty Images

So, what happened to the Boston Celtics? After year after year of dominance in the category, the Celtics fell to 22nd in the league, allowing opponents to shoot 37.4% from behind the arc. Did the year-over-year variance finally catch up with them? Or are there deeper reasons for the drop off?

Let’s start with how many three-point attempts Boston is allowing and where opponents are getting them from. The Celtics allowed about the same number of attempts per game from 2019-20 to 2020-21. In both seasons, Boston was smack in the middle of the pack in attempts allowed.

In terms of where the shots were coming from, the profile was again remarkably similar in terms of attempts allowed per game. In both years, Boston allowed 7.6 corner three-point attempts. For 2021, that was 10th in the NBA in terms of fewest corner three attempts allowed. In both seasons, the Celtics allowed opponents to shoot 39.5% on what is considered the easiest three-point shot in the game. That was about middle of the pack in percentage allowed from the corner.

We can rule out corner threes as being the problem. Attempts and percentages were almost exactly the same for the last two seasons. And going back as far as five years, they were all in the same range, even while adjusting for the rise in overall three-point attempts. Whatever Boston is doing to defend and limit corner threes, it hasn’t changed much.

How about above the break? This one gets a little interesting. Once again, attempts were about the same (27 attempts per game in 2020 vs 26.5 attempts per game in 2021), but the percentage was drastically different. In 2020, Boston held teams to 32.9% on above-the-break three-pointers, which was 2nd in the NBA. In 2021, that percentage jumped to 36.9%, which was 21st.

So, we seemingly found which shot-location broke Boston’s three-point defense. Now, the question becomes: Why did opponents shoot drastically better on those shots from year to year?

NBA: Brooklyn Nets at Boston Celtics Gregory Fisher-USA TODAY Sports

Multiple times this year, the question of effort has been raised when it comes to the Celtics. When this question has been pointed towards the three-point defense, we’ve never got a clear answer from Brad Stevens nor his players beyond “We can do better at closing out to shooters”. But what does that really mean?

In 2021, the Celtics allowed opponents to shoot 16.1 three-pointers per game that were classified as wide-open per tracking data. That means the closest defender is 6 or more feet away from the shooter. That mark was 15th in the NBA. The year prior, Boston allowed basically the same number of these attempts at 15.9 per game, which was tied for 17th. Slightly more of these attempts allowed, but Boston was actually higher up the chart in terms of league-wide volume.

All around the NBA, the number of wide-open three-point attempts were up. Why? The answer for this one seems to be simple: players are tired. Playing essentially every other day, often with travel, has guys worn out. Closing out is hard and requires having the legs to do it. All across the league, guys aren’t getting out to shooters like they usually do, Celtics included.

Compounding the issue for Boston is the luck factor. Despite allowing a similar number of wide-open attempts per game from 2020 to 2021, opponents made Boston pay more this season than ever. Teams hit 40.3% of their wide-open triples this year vs 37% last season.

If you change the filter to open attempts (closest defender within 4-6 feet), the numbers tell a similar story for the Celtics. They allowed a few less attempts, but the percentage jumped up by over three percentage points from 32.2% to 35.6%.

Take it down to tight attempts (closest defender with 2-4 feet) and the numbers remain the same. Boston allowed teams to shoot 29.3% on such attempts in 2020, good for 10th in the league. In 2021 that figure jumped to 33.8%, which was 25th in the NBA.

NBA: Dallas Mavericks at Boston Celtics David Butler II-USA TODAY Sports

Where does this leave us with the Celtics three-point defense drop off? Sadly, nowhere definitive. It’s not effort, or the team would be allowing considerably more open and wide-open attempts. It’s not system, as opponents aren’t getting more of the easier-to-make corner threes.

It seems like after 13 years of turning luck on its head, things finally caught up with the Celtics this season. Opponents simply made more of the same attempts than they have in years past. That may seem like a copout and that there should be a more definitive answer, but there doesn’t seem to be one.

Boston preaches to close “hard and high” by running out to shooters and leaping at them. Maybe with an overly young roster and tired veterans, they weren’t closing quite as hard or quite as high. Maybe they were just getting to the shooter, but not really impacting the shots. But that doesn’t really seem all that probable.

In the weirdest season the NBA has ever seen, the variance monster finally broke one of the NBA’s most improbably consistent runs. 2021-22 will tell us if Boston’s three-point defense dominance is really over. Or was it just a blip, like so many other things with the 2020-21 Celtics season?

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