The Boston Celtics' Al Horford Conundrum

Every NBA team has a weakness.

For the worst teams in the league, including the Brooklyn Nets and the Los Angeles Lakers, it's talent. They just don't have enough good players to win consistently. For a pair of mid-rung playoff teams—the Memphis Grizzlies and the Oklahoma City Thunder—it's shooting. They can't space the floor because they can't hit from outside, and that inability ends up hampering their offenses.

Even the best teams have weaknesses; they're just narrower in scope. For the Golden State Warriors, it's a propensity for turnovers. For the San Antonio Spurs, it's specific pick-and-roll matchups on defense.

The Boston Celtics, like all these teams, have a weak point. It's rebounding, specifically on the defensive side of the floor. Boston ranks 27th in the NBA in defensive rebounding percentage, per NBA.com, having collected the board on 75.2 percent of their opponents' missed shots. That number had ticked up a bit since the All-Star break (to 76.6 percent, 13th in the NBA), but collapsed again to 74.2 percent over the last two weeks.

Boston having this specific weakness is not a surprise. The Celtics start Al Horford at center, and defensive rebounding issues have plagued his teams for a few years now.

Before Boston, it was the Atlanta Hawks, who finished 25th and 22nd in Horford's last two healthy seasons there, snaring 73.2 percent and 74.6 percent of available defensive boards during the 2014-15 and 2015-16 campaigns, respectively. That, more than anything else, led to Atlanta getting swept out of the playoffs by the Cavaliers in each of the last two postseasons. During the 2015 Eastern Conference Finals, the Cavs hammered Atlanta on the offensive glass, corralling the board on nearly 30 percent of their misses. Tristan Thompson alone snared 17 offensive boards in four games. During the second round of the 2016 playoffs, Cleveland did it again, devouring offensive rebounds at a 31 percent clip. This time, Thompson gobbled up 24 of his teammates' misses in four games, while Kevin Love added 15 of his own.

Horford wasn't the sole contributor to Atlanta's rebounding woes, but he did play a starring role. He totaled just 28 defensive rebounds across those two four-game sweeps, and the Hawks' team defensive rebounding percentage was significantly worse with him on the floor during both series (around six percent in 2015 and eight percent in 2016). Given that the Celtics seem likely to face the Cavs in either the second round or the conference finals of this year's playoffs, depending on how the final few games of the regular season shake out, Boston will need to address this weakness of Horford's if they're planning on making a deep playoff run or perhaps even crashing the Finals.

It's easy enough to say that the Celtics could find a way to minimize Horford's minutes in a series against Cleveland, but that's not realistic for two reasons. First, they don't really have the personnel. Their other true bigs—Amir Johnson, Kelly Olynyk, and Tyler Zeller—aren't exactly stalwart defensive rebounders. Olynyk is the only one of the trio with a higher defensive rebound rate than Horford, and just barely. Going small with one of them alongside Jae Crowder or Jonas Jerebko in the frontcourt doesn't seem likely to alleviate the issue, either.

Second, Horford is simply too integral to everything else the Celtics do for them to take him off the floor for long stretches in an important playoff series.

The Boston offense doesn't quite experience an Isaiah Thomas–style drop-off (minus-15.4 points per 100 possessions) when Horford leaves the floor, but a difference of 5.0 points per 100 is still significant, equivalent to the difference between a top-four offense and a league-average one. More than that, it's Horford—not Avery Bradley, Jae Crowder, Marcus Smart, or backup point guard Terry Rozier—who acts as the true secondary playmaker in Brad Stevens' offense. Many of the Celtics' best sets actually involve Thomas giving up the ball early in the possession, and then looping around the floor to work with Horford in a dribble hand-off pick-and-roll; it gives him a cleaner path to the paint than if he simply gets a high screen or isolates his man at the top of the key. Boston also counts on Horford as a fulcrum for the offense, catching on the block or near the elbows and surveying the floor to spot open shooters on the perimeter or cutters darting into the lane.

Horford's scoring burden has dipped a bit since shipping up to Boston; his current 19.7 percent usage rate is the lowest it's been since 2011-12 and is more than two percentage points lower than it was during the Mike Budenholzer era in Atlanta. At the same time, his playmaking burden has never been higher.

In Atlanta, the Bud-powered Hawks whipped the ball around the floor with the best of them, but Horford was more often a finishing or a connecting piece than a true playmaker. In Boston, that balance has shifted. Horford is averaging a career-high 5.0 assists per game and has blown past his previous high-water mark in assist percentage by making the shot-creating pass on 24.7 percent of his teammates' baskets while on the floor this season. His previous high was 18.6 percent two years ago, when the Hawks won 60 games and had the sixth-best offense in basketball.

In addition to the improved passing numbers, Horford has added another valuable element to his game: an even more dangerous outside shot. During the 2014-15 season, Horford attempted what was then a career-high 36 three-pointers. He connected on a grand total of 11 of them, one more than he had in his entire career to that point. He took things several steps further last season, hoisting 256 triples (3.1 per game, and nearly four times as many as he had attempted during the first seven seasons of his career) and connecting at a 34.4 percent clip.

This season, Horford again upped the ante. Boston Horford has launched from deep 3.6 times a night, hitting at a nearly 36 percent rate. Barring an unexpected cold streak or rest dates, he'll snap the career-high in threes made that he set last season, despite playing in 14 fewer games. His ability to step outside and not only take but make threes has allowed Boston to flourish by playing five-out offensively at pretty much all times. (Kelly Olynyk replicates this skill off the bench.)

And it's not just the offensive end of the floor where Horford carries great importance. Because of the way the Celtics construct their defense, Horford is not just the last man between the opposition and the rim; he's an active participant in a versatile, switch-happy scheme that requires everyone on the floor to be comfortable guarding multiple positions. His light, quick feet allow him to deftly maneuver in space, corralling ball-handlers before scampering back to his man, but also picking them up outright and challenging them on their way to the basket.

While his rim-protection numbers are not quite at the top of the league, he's firmly entrenched in the second tier with guys like Robin Lopez, Tristan Thompson, and Anthony Davis. Horford doesn't block a ton of shots, but he does a good job of getting himself in position to at least challenge or alter them—and much like hits and hurries do a better job of capturing the way a pass-rusher affects the quarterback than do sacks, challenges and defensive field goal percentage help us understand rim-protection slightly better than merely counting blocks.

And so, the Celtics face a conundrum much like the one that confounded the Hawks before them. Boston needs to have Horford on the floor. The depth and diversity of his skill set is so great, and his contributions so interwoven with the fabric of the team's success, that it would be extremely difficult to advance without him. But Horford also exacerbates what it likely the team's biggest issue, and the one most likely to gnaw at them against the opponent almost certain to stand in their way on the road to their desired destination.

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