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BOSTON — It was like a volcano, stirring, waiting to erupt.

The Dallas Mavericks were being introduced before their game against the Boston Celtics on Friday, and while Dwight Powell, Max Christie, P.J. Washington and Khris Middleton were being welcomed with light boos, an anticipatory rumble was growing with the TD Garden crowd.

And then came the explosion.

“And from Duke, No. 32, Cooper Flagg,” the public address announcer said, a raucous ovation drowning out the end of the name.

It was the latest homecoming for Flagg, the Newport native whose first NBA game in New England drew the same throngs of local fans as his game at Boston College with Duke last year did, and that his games in Portland with Montverde Academy brought to the sold-out Cross Insurance Arena and Portland Expo the winter before.

But this was different. This was against the Celtics, the team that Flagg grew up watching and grew up worshipping. This is the team he was brought up to model his game after. From the time he was born, Celtics fandom was ingrained into his DNA.

And on Friday, for the first time at any level, he was playing in their home.

“It’s incredible to be able to play here,” Flagg said after scoring 16 points in a 120-100 loss to Boston. “Obviously, this is the place I came as a kid and got to watch, so I think it’s going to be incredibly fun for the rest of my career to be able to come here and play in front of this crowd.”

And the crowd was thrilled to see him. Throughout the concourses and in the stands, blue was everywhere among all the green, with hundreds of Flagg jerseys dotting the crowd. There were Mavericks jerseys. Duke jerseys. Even jerseys for Maine United, Flagg’s former AAU team.

“Half our town will be here,” said Casey Hardwick, an Ellsworth resident.

Celtics superstar Jayson Tatum, who made his anticipated return from a torn Achilles tendon, took the headlines for the day. But Flagg had a starring role as well. The ovation removed any doubt.

“It was incredible. I’ve never heard a road player get a cheer like that, especially here in Boston. They tend to go the other way,” Mavericks coach Jason Kidd said. “I think that just shows the appreciation they have for Coop, and who he is today and who he will be as his journey goes forward.”

Flagg heard them, loud and clear.

“Having a lot of people excited to be able to be here and watch you play is special,” he said.

The night was always going to be special. Flagg’s interest, and eventually, passion for basketball is tied to the Celtics. On long car rides to AAU practices growing up, Boston’s games from the 1980s replaced music or license plate games as the in-ride entertainment. As Flagg’s celebrity has grown, the stories of Celtics game viewings have gone from a fun fact to part of his folklore.

“They were constantly on,” said friend Dawson Townsend. “I can’t tell you how many times the three (of us) — me, Cooper and (twin brother) Ace — have watched the Houston vs. Celtics (1986) Finals.”

The Celtics were more than a favorite team. They were a window into the glory of the NBA, and a big-time that in a small town like Newport might have seemed unreachable — until it wasn’t.

“As kids, those were the only games that we could afford going to,” Townsend said. “We idolized the Celtics growing up. Him being able to play at the Garden on a Friday night is just such a crazy thing. It’s a full circle moment for sure, something that kids dream of.”

Flagg acknowledged as much.

“It meant a lot. I try to take a moment to take a deep breath and take it all in. It’s a dream come true,” he said. “Just being out there on that court, competing and playing at a high level, it’s really special.”

Kidd knew Friday would be emotional for his star rookie. He also knew the 19-year-old wouldn’t need help calming down.

“This young man handles the big stage like no one I’ve seen,” he said. “I thought he did great.”

It wasn’t perfect. While his Montverde and Duke teams rolled to blowout wins in front of his ardent fans, this homecoming ended with another loss in a dreary season.

Flagg added eight rebounds and six assists but went 7 of 23 from the floor, and less than four minutes into his second game back from a mid-foot sprain that cost him more than three weeks, he went down clutching his foot after getting fouled and crashing to the ground.

“He was aggressive, he found his teammates,” Kidd said. “He hurt his foot early, but he’s a tough kid. … He was aggressive, he had (good looks) that just didn’t go down, and that happens, no matter if you’re in Boston or you’re in Dallas.”

It wasn’t enough to ruin the experience. Flagg has played 51 games in his NBA career. Game No. 1 in Boston, however, was in a category of its own.

“It was definitely a different feeling,” he said. “This type of experience was really cool. … It was an incredible environment, and an incredible place to play.”