One of the Boston Celtics Achilles’ heels under Brad Stevens has been their inability to consistently control the defensive glass. The last few years this was explained away by the team playing a tiny backcourt of Isaiah Thomas and Avery Bradley, in addition to not having a glass eater among their bigs. When Al Horford joined the Celtics in 2016, he brought with him a lot of skills, but a superior rebounding presence wasn’t one of them. Horford has played 82 percent of his career minutes as a center per Basketball-Reference. In all that time, he’s only averaged as many as 10 rebounds per game one time, in the 2012-13 season. Not exactly what traditionalists expect from a center.
There are a lot of reasons for this. Horford rarely gets offensive rebounds, which can help inflate the total rebound stats. As he’s aged, his game has become more perimeter based, both out of necessity and strategy. Over the last two years, roughly 30 percent of Horford’s field goal attempts have come from behind the arc. This is a result of a few things: Horford embracing his three-point shooting as a weapon, Horford aging and being more comfortable on the perimeter versus in the paint and, last but not least, Brad Stevens unlocking him as a primary playmaker. All of these combined make Horford a dangerous and complete player, but take him out of the mix on the offensive board.
On defense, the story is somewhat similar. In the Celtics switch-heavy scheme, Horford regularly ends up defending perimeter players. In addition, when Boston has played him alongside another traditional big, he’s always the one defending the more perimeter-based offensive big. This takes him away from the rim and often out of the rebounding action. Take a look at this clip from Game 3:
The Bucks got the Celtics all sorts of mixed up here. By the time clips starts, no one is really guarding the player they started with. But look where Horford is: he’s glued to Khris Middleton in the strongside corner. By the time Malcolm Brogdon shoots from the top of the arc, Horford has no chance at the rebound, which Parker snags along the left baseline. What is also important to note is that Horford is paired with Aron Baynes up front. This means he’s going to be guarding a perimeter player, which naturally takes him away from some rebounds.
Here is another example from the same game:
Horford and Baynes are again paired together and Horford is out on the perimeter with Middleton once again. Brogdon misses from the corner, but Bledsoe already has the perfect position and bats it out. If Horford goes over him, he picks up a needless foul. If he tries to beat Bledsoe to the spot, he leaves Middleton too early, which has been death in this series.
In Game 5, Stevens made a couple of lineup changes. The most obvious was starting Semi Ojeleye on Antetokounmpo (read more about that move here). The second, less immediately obvious move, was with the rotation as a whole. With Marcus Smart back, Stevens stayed with the nine man rotation. With Ojeleye starting, that meant someone had to sit. For Game 5, that someone was Greg Monroe. But what this move also did was split the big man minutes between just Horford and Baynes. Horford played 37:33 and Baynes played 10:27, as Boston played the entire game with just one traditional big on the floor. And this made a huge difference on defense. Not only did it allow Ojeleye to defend Antetokounmpo, which he did quite well, but it allowed Horford to hang out around the basket more. Check it out:
First rebound of the game. Eric Bledsoe takes kind of a wild shot, but Horford is right there to clean the glass. This is because he’s guarding Tyler Zeller, who rarely strays very far from the paint.
Bledsoe misses the jumper and Horford is right there to vacuum up the board. Again, he’s guarding Zeller and that puts him in prime rebounding position.
On his last rebound of the night, Horford is again the lone big on the floor:
Middleton and Antetokounmpo run a screen action on the left wing, which results in Middleton taking the jumper while Giannis crashes the glass. Somehow the Celtics don’t account for Giannis and he gets a completely free run. But Horford is hanging out on the right baseline, guarding Thon Maker in the corner. As soon as the shot goes up, he leaves Maker alone and gets right to the spot to snag the rebound. This is the kind of smart play Horford regularly makes, but rarely gets credit for.
Al Horford wasn’t signed by the Celtics to grab 14 rebounds a game, as he did in Game 5, but Boston will happily take it when he does. By playing him as the lone big, he regularly does more on the glass for the Celtics. This is due to being in better position to rebound because he’s guarding an opposing big. Or perhaps he knows he has to be the one to handle the boards. Or, most likely, it’s a combination of the two. With little margin for error with their own scoring woes, the Celtics can’t afford to gift teams extra shots. Boston’s initial defense is almost always good, but a defensive possession doesn’t end until you have the ball. When Horford plays as the lone big man, as contrarian as it may seem, the Celtics more often close out possessions with the ball.