Boston Celtics, Nylon Calculus

Nylon Calculus: What’s driving the Celtics regression?

The Celtics haven’t looked like an Eastern Conference contender. Which players have fallen off the most from last year’s performance?

The struggles of the Boston Celtics have been one of the most puzzling stories in the early part of this season. At 10-10 after 20 games, their hopes of making a title run are certainly not gone but systems like 538’s projected this to be a 54-win team coming into the season. Their current average margin of victory, plus-2.1 points, is what we would expect from a 45-win team.

Talk of making a trade for a big star seems to be cropping up earlier than usual, along with the accompanying push-back and patience-preaching from Danny Ainge in the front office. Clearly, the Celtics are not what we expected them to be this year, particularly given the increased expectations after last season’s Eastern Conference Finals run. But what’s driving the decline?

ESPN released their first batch of Real Plus-Minus figures for the 2018-19 season and if we compare players marks from this year to last year, we can see which players appear to have regressed the most.

Real Plus-Minus is an estimate of a player’s impact at each end of the floor, expressed as a point differential per 100 possessions, based on both box score statistics and on-off splits. It’s not a perfect metric but it’s one of the better overall estimates out there and what it shows in the graph above tracks closely with other observations and evaluations.

The biggest regressions for the Celtics have been Al Horford, Jayson Tatum, Jaylen Brown and Terry Rozier, with all of their slides coming mostly on the offensive end. Brown’s decline is the most noticeable, going from a slightly positive offensive player to one of the worst in the league by RPM’s estimation.

Myriad factors have contributed to the overall breakdown of the Celtics offense, and shot-selection and shot-making have been identified as some of the major issues. Jaylen Brown is perhaps the most striking example of the latter. Last season, he made Brown made 39.4 percent of his 269 open and wide open 3-point attempts (no defender within four feet of him). This year he’s hit just 17 of his 72 (23.6 percent) of his open and wide open attempts.

As crazy as it sounds, his misses alone can explain a huge portion of Boston’s regression. If he was hitting open 3s at the same rate as last season, we’d expect him to have made an extra 11 shots out of his 72 attempts. All else being equal, those 33 extra points would be enough to 1.6 points per 100 possessions onto Boston’s offensive efficiency, moving them from 27th in the NBA to 23rd. Of course, all else is not equal. Brown being less threatening on the perimeter allows defenses to be more comfortable sagging into the paint, walling off drives.

Brown’s shooting probably feels so catastrophic, in part, because he was so accurate last season. However, last year’s overall 39.5 percent 3-point shooting was a huge jump from the 34.1 percent he shot as a rookie, the 29.3 percent he shot in his lone year at Cal, or the 32.6 percent we get as a rookie projection using Andrew Johnson’s formula and his college numbers. Some research has shown it may take a sample of around 750 attempts to get a stable measure of a player’s 3-point shooting ability, a total that Brown likely won’t hit until near the end of this season. It’s possible that his high mark last year as nearly as much of an aberration as this year’s low mark and that his “true” 3-point shooting ability is closer to the 34.0 percent range he shot as a rookie.

Next: Nylon Calculus Week 5 in Review: Exciting rookies and fun stats

Regardless of where Brown’s ceiling is as a shooter, it’s safe to assume it’s much higher than what he’s shooting right now and that at some point his percentages will climb. However, we’re probably past the point of the season where simply counting on progression to the mean, for Brown and some of their other struggling shooters, is going to be enough for the Celtics.

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