On the same roster with Kyrie Irving, Jaylen Brown, Marcus Smart, and Terry Rozier — well, Boston’s fifth guard is not going to get a lot of minutes.

So when Brad Stevens reached into his toolbox and announced Shane Larkin would start in place of Kyrie Irving against the Chicago Bulls, heads turned.

Perhaps they shouldn’t have. And we’re not just talking about Stevens’s post-game observation that with Larkin leap-frogging to starter, there’d be less disruption to a Celtics bench unit that has started to, well, become a unit.

Larkin has 39 other career starts under his belt with other NBA teams. (Okay, the Knicks and the Nets — literary license defense.)

But since joining the Celtics — like many players — Larkin seems to be upping his game. Shall we name this the “Brad Stevens Theory” (A contrapositive counterpart to the Bill Simmons cum Dave C. “Ewing Theory”?)

Call it what you want. Since getting acclimated to a new system and new teammates, Larkin has shown steady improvement.

From Dec. 1 forward, his true-shooting percentage, offensive rating, 3-point shot, and scoring are all up, as noted above. Even Larkin’s free-throw shooting has improved. He’s knocking charities down at an .864 clip, compared to about .760 prior to joining the Celtics.

As well — we don’t even like to say it — but injuries are a way of life in the NBA. So is the need for rest through the 82-game grind.

Given all this, it’s nice to have a guy like Larkin as your 11th man. Think about that. Shane Larkin is, more or less, only the fifth best guard in Stevens’s arsenal.

Shane as a Celtic – fun facts (at least until March 8)

Now that the serious stuff is out of the way, here are some fun facts that may not last long. (Although, considering he’s not likely to start against the Timberwolves, some of them may have some shelf-life.)

— As a Celtics starter, Larkin has a true-shooting percentage of 1.500, the highest mathematically possible, and, of course, an NBA record for starts with one team.

— When playing in any lineup combination that involves Al Horford (don’t scoff; it’s more than one-third of his minutes), the Celtics are more than 14 points better than their opponents per 100 possessions. That’s a lot. Larkin averages more than 20 points and nearly 10 assists per 100 possessions in those situations. It also illustrates the gravitational value of the NBA’s Rodney Dangerfield — namely, Horford.

— Again, limited sample size alert, but: Larkin has an ORtg of 201 for the entire month of March, or, if you prefer, as a starter for this season. (It’s one game either way.)