NBA Stats Notebook: Why Al Horford is the Celtics' playoff X Factor

NBA Stats Notebook: Why Al Horford is the Celtics' playoff X Factor

Professional basketball players are in a constant state of data collection. Al Horford's been logging tricks of his trade for 16 years, and it's helped him reinvent his game to suit a team in the title hunt.

Horford, age 36, will be the fourth-oldest starter and fifth-oldest rotation player in the 2023 NBA playoffs, behind only LeBron James, Chris Paul, PJ Tucker and Kyle Lowry. He has dramatically transformed his game to suit the era and needs of today's league, and the Celtics have taken note.

After a March 1 win against the Cavaliers, coach Joe Mazzulla alluded to Horford's evolution, heightened by his experience and attention to detail.

“It’s massive,” Mazzulla said postgame. “He’s done a great job this year in recognizing coverage and screening. ...And he recognized the coverage [the Cavaliers] were in and popped and shot it. The second time they tried to switch him, he created an advantage. He’s constantly trying to find small ways to reinvent his game.”

Horford is averaging his highest minutes per game (30.6) of the last five seasons, but his on-ball role has diminished to career-lows according to practically every go-to metric (usage rate, touches, touch time, etc.). He's played five total seasons with the Celtics across two stints; comparing his usage across those years is fascinating. Here's some touch data from PBP Stats highlighting three such years: his All-Star season in 2017-18, his first season back in Boston in 2021-22, and this current one.

Year Touches/game Frontcourt touches/game Elbow touches/game Post touches/game Seconds per touch
2017-18 63.5 40.0 3.1 5.2 2.20
2021-22 49.2 29.4 2.2 1.9 1.73
2022-23 45.7 25.4 1.1 0.9 1.53

It makes sense that Horford would take a back seat in his return to the Celtics given that Jayson Tatum and Jaylen Brown had risen to stardom. But the elbow and post touch volume in particular is stunning — Horford is getting the ball in those spots not even half as much as last year, and fractionally compared to his last All-Star appearance. His days of facilitating and scoring out of the post are over.

We all know Horford has laid down a transcendant shooting year, but the scale of his shift to the arc cannot be overstated. Per Cleaning the Glass, a whopping 67% of Horford's total shot attempts have come from distance. That is a mind-boggling 23% increase from last year; to put that in context, Horford had never increased his three-point frequency by more than 12% year-over-year since he first started shooting them often in 2015-16. 

To make this type of leap — and match it with a career-high 44.6% clip (an 11% jump from last year) — is one of the more astounding developments I've seen from a veteran in some time. 

Boston needs Horford at this level for the playoffs. Last year, the Warriors gunked up the Celtics offense in the NBA Finals by shutting off driving lanes for Jayson Tatum and Jaylen. They were unafraid to let Horford and Grant Williams bomb away above the break, even as both shot the ball well, if it meant Tatum and Brown could not penetrate the defense. 

But this year could be different. Horford can punish as a reliable outlet on his own or force opponents to stay home on him. He's also more than doubled his shot frequency in the corners and added an area defenses have to track.

Horford's evolving role means we see less of him as the facilitating big that has helped define his game in the past. But he's still a very good passer and the numbers (and tape) back this up.

The NBA Underground has a set of statistics that help us understand passing impact beyond basic assists per game. One interesting number: passing turnover to potential assist ratio, which basically says that for every 100 potential assists (pass leading to a shot attempt), a player is logging X amount of passing turnovers.

Horford is averaging 3.9 passing turnovers per 100 potential assists, putting him seventh-best among 364 players. It's not a perfect assessment of passing, but it shows he's making quality decisions when he does have the ball. Just under 10% of his total passes this year turned into assists or free throws, a mark that ranks ninth among bigs this year, per Second Spectrum. 

Plays like this are meaningful because Boston should want to get Tatum cutting and running without the ball. It keeps him from being the sole defensive focus and allows him to get up to speed more easily. Horford and Robert Williams III are both capable high-low passers, and while this isn't a jaw-dropping dime, it shows how either big can be leveraged effectively.

Horford isn't put in this position as often anymore, but he still gets plenty of assists by swinging the ball around at the arc and dishing short passes to Celtics guards and wings who use screens off the ball. There's schematic creativity on display around Horford and he's tasked with making the advantage pay off.

We could say the same for his defense. Derrick White has been Boston's best defender this season. Robert Williams is the team's most dynamic shot-blocker and Marcus Smart is the intense tone-setter. But I truly don't think this is a top-5 unit without Horford holding down his role as an individual stopper.

The Celtics typically task Horford with handling the opponent's best scoring forwards or bigs. Among the 20 players he matched up against for 20 or more possessions per game this season (per NBA Stats): Bam Adebayo, Deandre Ayton, Giannis Antetokounmpo, Evan Mobley, Jarrett Allen, Joel Embiid, Nikola Jokic, Paolo Banchero, Nikola Vucevic, Myles Turner.

Many of these players are also heavily involved in pick-and-rolls, where the Celtics usually ask Horford to play some form of shallow drop coverage while the guard goes over the screen. The shallow drop big man is one of the toughest responsibilities in today's NBA; they have to contain the drive from a guard while keeping the roll man in front to remove a lob threat.

And that's not all he's doing — Boston switches a lot away from the ball, so Horford then has to slide onto wings and occasionally guards. He actually spoke about this a couple weeks ago with JJ Redick on The Old Man and the Three (start at 31:30). 

Horford's impact shows up in several of the defensive stats I've come to trust. BBall Index has created estimates for defensive positional versatility and role versatility based on what positions and usage rates players are guarding. Horford ranks in the 92nd and 85th percentiles at his position, respectively. 

Also from BBall Index: Horford is contesting 38.1% of opponents' shots at the rim while he is on the court (89th percentile) and holding opponents to an at-rim efficiency 6.2% lower than expected (91st percentile). Those are particularly impressive numbers considering Horford is 36 and not a volume shot-blocker.

It will certainly help to have Rob Williams by his side. The two bigs have only played 330 minutes together this season, but the Celtics are outscoring opponents by 15.1 points per 100 possessions when they share the floor.

Still, Horford is going to take the primary frontcourt assignments before Williams. And this postseason run could be a gauntlet. The Celtics start by facing the Hawks' trio of John Collins, Clint Capela and Onyeka Okongwu. Win that series, and there's a good chance they meet Embiid in the second round. An Eastern Conference Finals trip could see Antetokounmpo and Brook Lopez, or Mobley and Allen. Then, should Boston make a return Finals run, they might run into any of Jokic, Domantas Sabonis, Jaren Jackson Jr., Deandre Ayton, or others at the big positions. 

That's a monster workload for a 36-year-old trying to play into June for the second consecutive year. And he carries pivotal pressure as a spacer and ball-mover. 

Horford's evolution has been commendable and his value to the Celtics far outweighs his spotlight. If Boston wants to win a championship, he can't let up on either end.

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