I have a theory (one less conspiratorial than that the current Paul McCartney is an impostor for the original and more innocent than San Antonio turned off the air conditioning to give LeBron James cramps in Game 1 of the 2014 Finals) that Marcus Smart doesn’t know how tall he is. Not for lack of measurement; he’s a professional athlete who has no doubt had his height taken countless times in the NBA, college, and high school. But Smart, who is listed at 6-foot-3, regularly guards players half a foot (or more) taller than him. He guards them, he bodies them, and by evening’s end, he owns them, bullying NBA bigs like he doesn’t know (or doesn’t care) that he’s only 75 inches high.

(Which is also part of the theory: History shows that the Celtics guard has never cared enough about any of his shortcomings to remember them. Shooting, for example. Smart will take four 3s on a Wednesday night and miss the lot of them only to shoot seven during Friday’s game. Friends, I ripped those numbers off his first two games this season. Spin the wheel on his game log, and you’ll find many similar instances. It’s Marcus Smart. Erraticism is canon.)

Celtics coach Brad Stevens also doesn’t care about Smart’s height. Stevens doesn’t care about the excess 3-pointers, either, thanks to Smart’s excellence on defense. The risk of Smart potentially shooting Boston out of a game is one most coaches would happily accept. Without Smart’s defensive valor in accepting the toughest assignments in the league, the Celtics wouldn’t be in the game to start. Smart is their only fail-safe against superstars. Come Curry, come Harden, come LeBron. Stevens trusts Smart with all positions. That faith in his frontcourt defense will be tested more than ever this season, with Al Horford and Aron Baynes gone, and with injuries already piling up. Smart is the best big-man stopper he has.

On Tuesday, Stevens tasked Smart with guarding Kevin Love, a 6-foot-8 power forward with an outside shot that should allow him to take advantage of height mismatches on the perimeter, and a 251-pound frame that should allow him to body smaller opponents in the post. Should. Love scored just eight points while Smart was defending him, which was 61 percent of the time, and overall he finished with 17.

Another theory: Love knew how Tuesday’s game would go. He was teammates with LeBron for four years, and watched Smart, who was voted first team All-Defense last season, thwart the best player in the world many times. “He’s really tough out there,” Love said after the game. “He can guard anybody, 1 through 5.” (Smart calls himself a “stretch-6” because of his ability to do so.) Last week, Giannis Antetokounmpo, who can truly play 1 through 5, was Smart’s assignment. Antetokounmpo is a multi-man job, yet in the 17 possessions Smart defended him, the Greek Freak scored just once.

Boston needs Smart to dominate these matchups. Al Horford’s and Aron Baynes’s departures this summer drained the blood from Boston’s center rotation. The Celtics picked up Enes Kanter, re-signed Daniel Theis, drafted Grant Williams, and signed Tacko Fall to join Robert Williams. That group didn’t exactly inspire the most confidence initially, and the Celtics’ frontcourt situation has worsened with injuries. Kanter has missed the past five games with a left knee contusion, and is questionable against the Hornets on Thursday; Theis and Williams are listed as day-to-day with ankle and hip injuries.

Let’s pretend for a moment that the frontcourt is at full health: Kanter is the starting center, though he has the least defensive potential of the group. Theis, having been in Boston for two seasons, is the most acclimated to Stevens’s system. Each younger player has upside. Theis demonstrates mobility and shot blocking. Williams has the NBA’s two most coveted qualities you can’t buy: length and athleticism; I’m convinced that he has a pull-up bar on every door frame in his house. At 6-foot-6, the rookie is undersized, but makes up for it with strength. (He won the bro-iest exercise at the 2019 combine, benching the most reps.)

Against true centers, the Celtics don’t have an answer on defense. But for any player with “stretch” in their descriptor—the lengthy, combo forward scorers that Horford used to extinguish—Smart is more than capable.

Smart is a master trespasser, both on the ground and in the air, which allows him to match up with such larger players. He picks up on the natural pauses that a ball handler will take—looking up for a potential pass, holding the ball above one’s head to scan the court—and pounces, swiping it through the hands of whatever poor, unwitting opponent he’s been bothering for the past 20 minutes. Like the Clippers’ Patrick Beverley, Smart works his way into his opponent’s space, blocking avenues with his lower body, until suddenly, the player finds himself trapped in Smart’s space instead.

While he’s never not ready to jump another player, Smart also knows to wait for the moment the handler is most vulnerable. When a player is gathering for a shot underneath the basket, for example, Smart often swats the ball at its lowest point, before his man has gained any upward momentum. Vertically, Smart has little problem intercepting passes, or reaching taller players trying to play keep away in the air. His standing leap at the combine was ninth best in his 2014 draft at 33 inches, one spot better than Aaron Gordon, the man who soared high enough over a mascot to place the ball under his legs before dunking it.

Smart doesn’t just deflect passes, he also recovers possessions like few others. There isn’t a stat for the percentage of times one player wins the ball wrestling it away from another, but if there were I’d bet money Smart would lead it. He doesn’t flinch, doesn’t hesitate to dive on the hardwood, doesn’t mind if you think he’s flopping. (He is.)

When Boston re-signed Smart in 2018 to a four-year, $52 million contract, it took the team over the luxury-tax line, seemingly a questionable investment for a player who is a near zero on offense. But the front office knew it needed Smart, now the longest-tenured Celtic, to hold together the defensive identity Stevens had introduced. And to think, the term “stretch-6” didn’t even exist back then.