BOSTON — Seventeen years ago, Josh Longstaff was among the fans chanting “Let’s Go, Celtics!” at an NBA playoff game at the FleetCenter.

Back then, Longstaff was a college freshman at Bryant University, a walk-on to the basketball team after playing at Portland High. He cheered for a Celtics squad that included Paul Pierce, Kenny Anderson and Antoine Walker.

“I loved the game,” said Longstaff, “and the experience was really cool.”

On Friday night, Longstaff was back in the same building – renamed TD Garden – for another playoff game. Now 36 and newly married, he’s in his first season as an assistant coach for the Milwaukee Bucks. With his parents and brother in the stands, Longstaff sat on the back bench, a row behind head coach Mike Budenholzer, and exhorted the visitors from Wisconsin.

The Bucks defeated the Celtics, 123-116, to take a 2-1 lead in the Eastern Conference semifinals. Game 4 is scheduled for Monday night in Boston before the best-of-seven series returns to Milwaukee.

Milwaukee assistant coach Josh Longstaff has a word with Bucks star Giannis Antetokounmpo during All-Star festivities in February in Charlotte, N.C. Nathaniel Butler/NBA

“I obviously don’t want to say anything to anyone,” Longstaff said after the Bucks’ workout earlier Friday at Boston University, “but to me, this is so special to be part of a playoff series against the Celtics. The playoff atmosphere (in Boston), you can’t really prepare for it, and I think the players understand that, too.”

Longstaff broke into the NBA with the video department of the Oklahoma City Thunder in 2010 after five years as a high school coach in Maine. He later spent three years as a Knicks assistant before becoming a G League head coach with the Erie BayHawks in 2017-18. Last summer, Longstaff got an offer from Budenholzer, who had moved on to Milwaukee after five seasons with Atlanta. The Hawks were the parent club of Erie, so Budenholzer was familiar with Longstaff.

“We all have lists, and he’s been on my list for a long time,” Budenholzer said Friday. “With what he did in Oklahoma City and the culture they have there and their player development, he was just somebody I always thought would be great to add. He was tremendous in Erie last year, and now he’s been even better with us every day.”

Longstaff isn’t the only G League and Portland Expo alumnus involved in this playoff series. Scott Morrison, head coach of three straight Red Claws playoff teams, is in his second year as an assistant to Celtics Coach Brad Stevens. The most electrifying player during Morrison’s Red Claws tenure, Tim Frazier, signed with the Bucks in March after being released by New Orleans.

The three of them came together last week in Milwaukee. Morrison, who loves to needle, strolled up to Frazier and said, ‘Tim, you know you’re not the best point guard to ever play in the Expo.’ When the former D League MVP looked up in surprise, Morrison raised his eyebrows and pointed a finger toward Longstaff.

“It was a running joke between me and Scott,” Longstaff said. “Tim gets a kick out of it every time.”

Frazier, 28, has managed to carve out a five-year NBA career as an end-of-bench substitute who takes nothing for granted. He said he and Bucks guard Pat Connaughton are under the personal tutelage of Longstaff, who directs their pregame workouts.

“He knows his stuff,” Frazier said of Longstaff. “He’s very detailed in his work. He pushes me, makes me work hard.”

Bucks guard Pat Connaughton, who grew up in Massachusetts, converses with assistant coach Josh Longstaff, a Portland native. Nick Monroe/Milwaukee Bucks

Connaughton had 14 points and seven rebounds off the bench on Friday night, connecting on four 3-pointers.

“Just from him being from Maine, we hit it off as northeastern guys,” said Connaughton, who grew up in Arlington, Massachusetts.

“Obviously he’s great as a basketball coach, but it hasn’t just been basketball that we’ve talked about. In the NBA, that’s huge. You want to be able to develop guys from a basketball standpoint, but there’s also a bunch of different personalities, a bunch of different egos that you have to be able to manage.

“Guys respect him. Guys feel like they can talk to him.”

Longstaff has a knack for connecting with stars as easily as he does with subs. While in Oklahoma City, he worked with Russell Westbrook, Kevin Durant and James Harden, and he reconnected with all three players in February because the Milwaukee staff coached Team Giannis at NBA All-Star weekend in Charlotte, North Carolina.

Another highlight was meeting Durant’s Golden State teammate Steph Curry.

“We actually hit it off a little bit because he’s best friends with Bryant Barr,” Longstaff said of the former Falmouth High grad who played and roomed with Curry at Davidson College. “We got to talking and I told him I was from Maine, and he was like, ‘I know where this conversation’s going.’”

If Milwaukee, as top seed in the East, can win twice more against the Celtics, Longstaff would find himself in the Eastern Conference finals, perhaps against a Philadelphia 76ers squad coached by South Portland native Brett Brown. It would serve as yet another reminder of Maine’s remarkably deep roots in the NBA coaching landscape, which includes head coaches Kenny Atkinson (Maine Central Institute) of the Nets, Jim Boylen (University of Maine) of the Bulls, Rick Carlisle (UMaine) of the Mavericks and Steve Clifford (UMaine-Farmington) of the Magic.

“I feel like we all approach things the same way – kind of hard-working and appreciate where we came from,” Longstaff said. “We talk about Maine all the time.”

The players, he said, find it interesting because aside from the occasional Red Claws alumnus, they have no connection to Vacationland.

“I just hope I’m representing what Portland, Maine, is all about and what the (Maine) people are all about,” Longstaff said. “It’s something that’s important to me and I know from speaking to those guys, it’s important to them, too.”