A lot of things have to break just right for a team like the Boston Celtics to be this good. After losing Gordon Hayward to injury in the first game of the season and starting 0-2, the Celtics have run off 14 straight wins — including a win Thursday over the reigning champion Golden State Warriors — on the strength of a suffocating defense and surprising contributions from young players like Jaylen Brown and Jayson Tatum. But while Boston’s success on defense has been a collective effort, the other side of the ball has in large part been buoyed by a single player. It just may not be the one you think.Kyrie Irving makes the Boston offense go. He leads the team in scoring, assists and usage percentage. He drives and dishes and shoots the Celtics back into games they ought to be losing, and he provides the piercing thrust to an offense that is long on shooters but short on shot creators. When the offense looks right, it’s usually Irving at the wheel. But as good as Irving has been, he isn’t the Celtic who’s had the biggest impact on the offense. That would be Al Horford. Related: The Lab Horford’s individual numbers don’t look much like a superstar’s. He averages 15.5 points, 9.0 rebounds and 4.4 assists per game. His shooting percentages are way up this season — 44.4 percent from 3 with a 68.1 true shooting percentage, both certain to cool at some point — but that doesn’t really account for his outsize effect on the team. The Celtics’ offense is 12.7 points per 100 possessions worse when Horford leaves the floor, more than six times the difference when Irving sits. That’s not just early-season noise: Horford affects the game in subtle but pervading ways, improving the outlook of plays in which he may never touch the ball.Absent box-score evidence, in the past these skills would be dropped into the catch-all of “making teammates better.” But today we can track how well players perform when Horford gets involved.For instance, Irving is by far the most effective pick-and-roll ball-handler on the roster, generating 98.8 points per 100 chances, according to data from Second Spectrum — far more than Marcus Smart (79.2) and Terry Rozier (83.2). But if we turn that around and look at how effective pick and rolls are based on who’s setting the screen, we can see Horford’s effect: When he sets the pick, Boston scores 103.7 points per 100 chances; with any other screener, it’s just 81.1. Horford’s numbers with Irving are great, as you would expect, but he also elevates Smart, Rozier and anyone else who can fog a mirror.This holds up away from the ball as well. While the Brad Stevens offense is known for its off-ball movement, Boston scores just 91.7 points per 100 chances created by off-ball screens, according to Second Spectrum — a good but not outstanding number. When it’s Horford setting the screen, though, that shoots up to 106.6 points per 100 chances. Compare that with Aron Baynes or Daniel Theis, the two Celtics who set the most screens aside from Horford, and it’s no contest. Baynes’ screens result in the Celts getting 79.1 points per 100 chances; Theis’s aren’t much better, at 88.8. In other words, it isn’t a system thing — Horford is producing this effect.What sets Horford’s screens apart is how long they last. He doesn’t simply brush up against the defender and sprint off to his spot on the floor — he maintains contact as long as possible, turning, bending and reaching to stay in the defender’s way, then beginning his roll to the rim while the defender is still on him, often dragging him along for a step or two. These aren’t exactly moving screens (at least not all of them), but Horford gets in the way and stays in the way. Couple that with the attention his shooting demands this season, and Boston guards either have a clear path to the rim or can kick out to one of the best floor-spacers in the game.This has a cascading effect. Irving and Smart have both created high-value shots for teammates this season, but both are aided by defenses scrambling to catch up to cutters or drivers sprung by screens. Yet when Horford makes the pass himself, teammates have a 51.3 effective field goal percentage — lower than might be expected. His overall effect on teammates’ shots isn’t as strong as Irving’s or Smart’s, though his numbers are weighed down by being unable to pass to himself and by passing to Smart, who is by many standards the worst shooter in the NBA today. (Irving has passed to Smart for a shot just four times this season.) Irving has a 65.5 eFG on passes from Horford, up from 48.5 percent overall; Brown and Tatum remain about the same, while Smart’s ridiculous 37.5 eFG when Horford gets him involved is an improvement on his heinous 32.4 mark overall.Of course, this early in the season it’s hard to say how much of this will last. Tatum and Irving are making difficult shots at a rate that’s hard to imagine them maintaining; Horford is shooting like Kyle Korver for the first time in his career; and even the vaunted defense is built on a little more luck than it may let on.(The Boston defense is allowing just 97.2 points per 100 possessions, putting it in hailing range of the 2003-04 San Antonio Spurs and Detroit Pistons for best full-season defense in the modern era.1Excluding lockout-shortened seasons. Its defensive effective field goal percentage is a league-low 47.7 percent. According to Second Spectrum’s quantified shot quality, which takes into account shot location and distance, proximity of defenders and other variables, the Celtics allow opponents the lowest value shots of any team in the league. But Boston’s opponents’ shots are depressed event further, by the second-highest rate in the league. That could be thanks to the aggressive challenges Boston makes along the perimeter, but it could just as easily be a fortunate run to start the season.)At age 31, Horford is oldest member of a Boston roster built for the future. There’s no guarantee he’ll remain this effective by the time Brown, Tatum and the rest of the young team come into their primes. But windows of contention can be forced open earlier than expected if the young bucks can fill their roles and a few key veterans hold down the rest. So far, Horford has been up to the task.Check out our latest NBA predictions. Can The Celtics Keep On Winning?