What Happened to the Boston Celtics Defense?

At the season's halfway mark, we have enough information to see what exactly Boston needs to improve on.

The Celtics have a much better record halfway through this season than they did last year on the back of the historical fourth quarter performances by Isaiah Thomas. Yet, even when healthy, the Celtics have a noticeably worse defense than they did last year. Have they lost a step, or is it connected to the number of gargantuan individual performances that other players are putting on this year? I won’t hesitate to lay out criticism where it is due, but is there a single defender or even a defensive scheme that can restrain the likes of Russell Westbrook, James Harden, and Demar Derozan? Probably not, but the numbers show that some Celtics aren’t pulling their weight on the defensive end.

If you look at points per possession, the Celtics only give up 0.935 each trip, which is the eighth best rating in the league. However, the Celtics will often allow their opponents more possessions than they deserve through careless turnovers, giving up offensive rebounds and stealing the ball less than they used to.

In transition, the Celtics move up to the second best rating at 0.995 points given up per possession. This is great but only accounts for 13.6% of all defensive possessions this year. By the eye test, I would call this a “hustle stat.” The Celtics can run with anyone and even if it doesn’t always pay off, they contest the majority of the shots.  It would take a loud finisher like LeBron to score in transition against the Celtics.

So, about that other 86.4% of defense that gets played. Boston ranks 18th at 0.944 points per possession in the half court. Hovering around the league average with a group that is considered to have some elite defenders at multiple positions is not ideal. Even worse, some stats would show that some of our best defensive players have been liabilities in some situations. Looking at PPP again, the Celtics are dead last in defending isolation plays.

On isolation plays, some of the lowest rated Celtics are Al Horford (1.216 PPP), Marcus Smart (1.111 PPP) and Avery Bradley (1 PPP). Those three players are all in the top four of those on the roster who have had to defend the isolation the most, the fourth being Kelly Olynyk, who is in the top half of the league at 0.833 PPP.

I like to take a more theory-based approach to figure out why things are how they are instead of looking at specific instances of bad defense. I could bury my face in my computer screen, watch our players get toasted, and say “yep, that was a clip of Al Horford getting scored on.” I’m skipping the how things happen and trying to figure out why. I know the “how” already. Horford’s man is usually going to back him down and either spin off him for an easy two or take a hook shot. Some centers have the range to pull up for a midrange shot, but that’s proven to be risky against Horford’s length.

With Bradley and Smart, it’s often a case of trying to allow as little separation as possible so that somebody like Russell Westbrook can’t get off any shot he wants. As a guard, you should be ready for all the classic moves to come out – jab steps, crossovers, shimmy-shakes, and pump fakes. Westbrook can do them all.

What I’m getting at is that I don’t think it’s purely coincidental that a year marked by great individual performances is also a year where the Celtics get crushed in isolation. For every bit the Celtics are gritty, they are also undersized and often outmuscled. Isolation offense accounts for about 7.3% of what the Celtics have had to defend this year, but I don’t care how small that number gets if it represents what Demar Derozan is going to do if the Raptors meet us in the playoffs. That said, 7.3 is a relatively small number no matter how you look at it, so there have to be other factors to the Celtics’ struggling defense.

Another small one is the hand-off. It only accounts for 3.5% of the Celtics defensive possessions, but it’s another area where they rank last in the league. The hand-off is often a pick and a pass given to one player simultaneously, usually a big man handing the ball off to a scorer while blocking the defender’s path. The most victimized Celtics in this category are Isaiah Thomas (1.167 PPP), Terry Rozier (1.409 PPP) and Jaylen Brown (1.6 PPP, but on far fewer possessions). I’m not one to target Thomas as a defensive liability, but in this instance, it’s a matter of a small person trying to run around a big person. Whether Isaiah is able to fight over picks or quickly slide under them, it’s not a play designed for people of his stature.

Still, there’s one more problem to address that I can’t put off any longer.


I no longer have any positive spin I can put on rebounding. Last night, the Knicks outrebounded the Celtics by 24 without Kristaps Porzingis or Joakim Noah available to play. If the Celtics haven’t found an answer to their rebounding problem 42 games into the season, then something simply has to change. Not only because Derrick Rose’s 10 rebounds exceeded the rebounding total of any individual Celtic, but also because the Celtics are 29th in the league in total rebounding percentage, sandwiched in between Brooklyn and Dallas. The Celtics pull down only 47.2% of available rebounds, which separates them from the top half of the league by only about three percent.

We’re past the point of saying “they just need to box out.” They know how to box out, and yet opposing players will swoop in for second chance points while the Celtics watch the ball careen off the rim. Sure, one can expect a decline in rebounding when one of Boston’s best rebounders has missed some games to injury, but you can’t be happy about the reliance on Avery Bradley to compete on the boards.

The lack of rebounding helps to explain why only looking at points per possession won’t tell the full story. Even if they can guard each possession relatively well, or at least around the league average, the sheer number of possessions they surrender on the boards begin to add up.

Should Danny try to trade for a rebounder? Should we wait for Ante Zizic to come over next year and see what he can do? My vote is to stay the course and continue to develop young players with high ceilings. I’m confident that the Celtics’ management will build something great, but part of that means the team will inevitably look a lot different when things come together. If Danny can ship out Tyler Zeller and second round picks for a rebounder in the short term, then I’m all for it. Otherwise, there is no need to give up anybody of significant value for a band-aid solution.

The NBA has evolved so that much of the game takes place in the paint and beyond the arc. It may be the case that the Celtics have not adapted to this change, and that’s why they give up a lot of points to explosive scorers and strong post players. They can still clog the passing lanes and block shots, but those strengths simply do not cover up their weaknesses against quality opponents, or apparently, the Knicks.

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