Something is off in Boston. The Celtics are stuck in fifth place in the Eastern Conference with a 25-18 record, despite having one of the most talented rosters in the NBA. The players don’t seem to particularly like each other, and Boston head coach Brad Stevens hasn’t been able to get them on the same page. It doesn’t help that the one guy who connects them all together has been missing in action. Al Horford has been showing signs of age while dealing with a lingering knee injury that kept him out of seven games in December, and the Celtics have been managing his playing time carefully since he came back. He is the key to their hopes in the playoffs. A healthy version of Horford would solve a lot of their problems.

Horford hasn’t been himself over the first few months of the season. He looks mostly the same on offense, though he isn’t rolling to the rim as much. But he’s still knocking down 3s (34.9 percent on 3.2 attempts per game) and making plays for his teammates (3.9 assists per game) while taking care of the ball (1.5 turnovers per game). Horford is a savvy player who is rarely out of position on either end of the floor. The difference is that he’s not getting there as fast as he once did. He just seems a step slow at times, which shouldn’t be a surprise for a 32-year-old in his 12th season in the league.

His defensive numbers have slipped across the board. The Celtics have been better defensively this season with Aron Baynes and Daniel Theis at center. Their defensive rating drops from 104.2 in 946 minutes with Horford to 98.5 in 523 minutes with Theis and 97.3 in 356 minutes with Baynes. They are still a top-five defense with Horford, but he’s no longer driving their success. He has one of the worst defensive ratings of any player in their rotation. The decline isn’t coming in any one area. Horford has been worse while defending almost every play type this season than he was in his first two in Boston, according to the tracking numbers at Synergy Sports:

Horford Defense Horford Percentile on Defense 2016-17 2017-18 2018-19 Horford Percentile on Defense 2016-17 2017-18 2018-19 P&R Ball-Handler 38 79 54 Spot-Up 70 84 55 Post-Up 94 76 30 P&R Roll Man 55 62 41 Average 64.25 75.25 45

It’s hard to know what exactly is going on. The Celtics may just be saving him for the playoffs. He is averaging a career-low in minutes (27.8 per game), and they have been doing everything possible to limit his workload. He is averaging 23.9 minutes in the 12 games since he returned to the lineup on December 23. He has exceeded 30 minutes only once in that span. It’s unclear whether he even had a specific injury. He never went down in a heap, but did complain about knee soreness before being diagnosed with patellofemoral pain syndrome, the medical term for “runner’s knee.” He might just be getting old.

He has put a lot of miles on his body. He has played in three of the last four Eastern Conference finals, going back to his time in Atlanta, and he is no. 34 among active players in career minutes (24,758) played. His peers are starting to age out of the league. Joakim Noah, whom he won two NCAA titles with at Florida, is a shell of himself. While Horford has taken better care of himself than Noah, and he never played for a coach as hard-driving as Tom Thibodeau, he is still one of the smallest starting centers (6-foot-10 and 250 pounds) in the NBA. Bigger centers tend to last longer than ones who depend on speed and athleticism. Horford has given up his body to battle in the paint for more than a decade, and he’s had to do it more often than fellow small-ball big men like Draymond Green, who plays only spot minutes at center. How Horford ages will be an interesting test case for the durability of this new generation of small-ball 5s.

The Celtics need him to find the fountain of youth at some point. His impact on offense goes far beyond his limited scoring numbers (11.5 points per game on 50 percent shooting). His presence gives structure to everything else they do. There aren’t many centers who can stretch the defense and make plays from the perimeter. Horford is one of only six, along with Anthony Davis, Nikola Jokic, Nikola Vucevic, Joel Embiid, and Marc Gasol, averaging at least three 3-point attempts and three assists per game this season. He sets screens, moves the ball, and spaces the floor for everyone else, relieving some of the pressure from the guards and allowing them to hunt for their own shot.

Boston doesn’t have an elite playmaker. Kyrie Irving is playing the best basketball of his career, and he’s still averaging only 6.4 assists per game. He’s at his best when he is in attack mode, putting pressure on the defense by taking and making impossible shots off the dribble. It’s the same story for Jayson Tatum and Marcus Morris, their second- and third-leading scorers, who average fewer assists combined (3.2) than Horford does by himself. Playing Horford at center is like sneaking an extra guard into the lineup in a place where the defense isn’t prepared for it. He forces teams to adjust their defensive principles. The Bucks drop Brook Lopez all the way to the front of the rim when he’s defending pick-and-rolls, but they can’t do that against a pick-and-pop center like Horford.

What makes Horford special is that he combines his offensive impact with the defensive versatility to protect the rim and extend out on the perimeter. Look at the list of big men with his skill set. Jokic and Vucevic are not prototypical rim protectors, while Gasol and Embiid struggle to defend in space. Horford exposed Embiid in the second round of last season’s playoffs by forcing him to defend at the 3-point line. Davis is the only big man in the league who can replicate what Horford does on offense without taking anything away on defense.

Horford is why people think Stevens is a magician. The sixth-year head coach lost in the first round in his first two playoff appearances without Horford. Having a center who created matchup problems on offense without being targeted on defense allowed the Celtics to punch above their weight in the 2017 and 2018 playoffs. The same thing happened when Horford was with the Hawks. The problem, of course, was that he never had an answer for LeBron James. The story of the Eastern Conference playoffs over the last four seasons has been Horford beating everyone in his path before losing to LeBron. His teams are 7-4 in that span, with all four losses coming to the Cavs.

Horford is a force multiplier who makes everyone around him better. Isaiah Thomas averaged 21.5 points on 37.6 percent shooting in two playoff trips before Horford and 23.3 points on 42.5 percent shooting in one trip with him. Terry Rozier, who could wind up losing playing time on the Celtics’ second unit to Brad Wanamaker, may never play as well as he did next to Horford (16.5 points on 40.6 percent shooting and 5.7 assists per game) in last season’s playoffs. Horford’s unique skill set is what allowed the Celtics to survive without Kyrie and Gordon Hayward. They don’t have anyone who could replace him.

Boston could get by on defense with a platoon of Theis, Baynes, and Jaylen Brown as a small-ball 5, but there would be a gaping void on offense. Defenses don’t have to respect Baynes and Theis on the perimeter. Even when those guys make 3s, they can’t attack off the dribble and make plays on the move like Horford, so opposing big men can stay closer to the basket and close-out much harder on them. They certainly can’t be the hub of the offense in the high post. Not even a wing like Brown could handle all of Horford’s playmaking responsibility. Horford has averaged 34.8 minutes per game over the last two playoffs. The Celtics will need him back in that range to live up to expectations.

He is the difference maker in any potential matchup with the other elite teams in the East. Horford creates issues for Embiid (Philadelphia), Lopez (Milwaukee), Myles Turner (Indiana), and Serge Ibaka (Toronto). Those guys are not as comfortable defending on the perimeter as Horford, and a huge part of their team’s defensive schemes revolves around their big men protecting the paint. There’s no obvious counter to a player like Horford, either. Moving a center off Horford forces them to chase an elite wing around the 3-point line. And no team in the East has as much wing depth as Boston in a small-ball game.

Those factors won’t come into play if Horford can’t get back to where he was in last season’s playoffs. He still has to be able to slow down a guy like Embiid enough to the point where Embiid’s defensive shortcomings will become an issue. He won’t win wrestling matches at the rim. He has to beat Embiid to spots and set up before he gets there. And if he isn’t going toe-to-toe with a bruising giant like Embiid in the playoffs, he’ll have to keep up with either Pascal Siakam or Giannis Antetokounmpo, two of the fastest frontcourt players in the league, at the 5. Losing even half a step might be the difference in any of those matchups.

The window in Boston may not be as open as it seems. Horford doesn’t have that many good years left. He won’t age as well as a legitimate 7-footer like Tim Duncan. The Celtics have been preparing for his decline by stocking up a war chest for Anthony Davis, but there’s no way to know where the Pelicans superstar will wind up. Winning the Davis sweepstakes may be a necessity for them, not a luxury. Their younger stars might never be as good as Giannis or Kawhi Leonard. The Celtics will need a big man who can help their other players punch above their weight. They had better make the most of their time with Horford because there’s no guarantee they’ll get someone better.