ONCE AGAIN, BRIAN SCALABRINE finds himself on a court working on his game. But this time, instead of a basketball, he’s holding a racket and trying to hit a tiny rubber ball that barely bounces. Since being elbowed into retirement last year, the 11-year NBA veteran whose popularity has long dwarfed his playing time has taken up an unlikely sport: squash.

On this January morning, the 6-foot-9-inch Scalabrine is in the middle of very small tempered-glass enclosure at the ritzy Tennis and Racquet Club on Boylston Street. He’s dripping with sweat, dashing back and forth across the court with enormous strides. He’s playing hard — and losing. In fairness, his opponent is Shahid Zaman Khan, the club’s 5-foot-8-inch resident pro, a scion of a Pakistani squash dynasty who was once ranked the 14th best player on the planet. Scalabrine picked up the sport less than a year ago.

Squash is a small but important part of the new Brian Scalabrine Game Plan. The ex-Celtic has reached the place where all pro athletes someday find themselves, aged out of the sport to which they devoted everything, yet with their whole lives still ahead of them. Some lose their shirts to hangers-on and poker tables; others slip into a quiet retirement, spending weeks on golf courses or the beaches of Bermuda. Not 34-year-old Scalabrine, who’s both too smart to blow his millions and too antsy to sit still for long.

Instead, he moved back to his home state of Washington, where he lives just outside Seattle with his wife and two young daughters. He still plays hoops with friends, though it’s tough to find competition, and he bought a longboard. Skateboarding lets him work on his balance, he says, a point he underscores by bending his knees and stretching out his arms. For work, Scalabrine landed a broadcasting job with Comcast SportsNet New England, putting his gift of gab to use as an in-studio analyst and part-time commentator for Celtics road games. He was terrible in the beginning, as he’s the first to tell anyone, but he prepared hard, asked questions, and learned. This is how Scalabrine approaches every new challenge. This is the secret of Scal.

 On the squash court, Khan scores another point. He’s now winning 5 to 1. Scalabrine lowers his racket and asks him a question. Like basketball, squash is a game of angles. Scalabrine sees those relationships on a basketball court, but some of the subtleties within this tiny space escape him. His strokes need work, and he frequently over-pursues the ball, chasing it too close to the back wall.

Unembarrassed by his struggles, he listens carefully as Khan explains how to hit a proper forehand. He learns fast. The next time Khan drives a ball to the back wall, Scalabrine returns the shot with noticeably improved form. It’s still not the prettiest thing in the world, but it gets the job done.

Story of his life.

***

NINETY MINUTES before the tip-off of the Christmas Day Celtics game against the Nets in Brooklyn, Scalabrine is the most popular man on the Barclays Center floor. As a woman who vaguely resembles Rihanna puts the Brooklynettes through their warm-ups, he chats with the Celtics’ assistant coaches, ESPN’s Chris Broussard, players from both teams, and a steady stream of well-wishers.

Everyone, it seems, wants to talk to Scal, and he’s happy to oblige. When he signs an autograph for a kid in a Joe Johnson Nets uniform, the boy is ecstatic; his father perhaps even more so. “It’s amazing,” says Mike Gorman, Scalabrine’s partner on the Comcast broadcast. In Chicago a couple of weeks ago, Gorman adds, “he was more popular than Rahm Emanuel.”

Scalabrine wasn’t always a cult hero. After an unremarkable high school career — he was cut from his freshman team — he stepped up in junior college, then built himself into a bona fide star at the University of Southern California, leading the Trojans to their first March Madness victories in two decades. The New Jersey Nets picked him in the second round of the 2001 draft, though he was plagued by injuries before the season even started. After years of struggling to make an impact, he scored a career-high 6.3 points per game in 2004-2005, then parlayed that into a five-year, $15 million contract with the Celtics.