With time running down in the third quarter and the Bucks’ 19-point lead erased, Jayson Tatum stepped back and hit a 3-pointer, and TD Garden erupted like it might during a playoff game.
As Tatum strutted back downcourt and celebrated, a fan in a courtside seat briefly stepped onto the floor to celebrate with him. But when that fan is Paul Pierce, it’s best that you join him.
“Just to see someone like him standing up, yelling, cheering,” Tatum said. “He told me, ‘Give me a five, give me a five’ . . . It was a great moment.”
The presence of Pierce, whose No. 34 hangs from the rafters here, provided a reminder of what these Celtics are ultimately pushing toward. He was a member of the last Celtics team to win a title, in 2008, and he symbolizes the gritty, hard-playing style that these Celtics hope to eventually emulate.
Even though it is just October, they took an important step in the right direction on Wednesday by clawing back from the big deficit against one of the NBA’s powers and grabbing this 116-105 win.
The last time the Celtics saw the Bucks, Milwaukee was flicking them away in five games in the Eastern Conference semifinals. The stakes were not as high this time, but Boston felt good about it nonetheless.
Throughout training camp, coach Brad Stevens sounded uneasy as he talked about the dangers lurking at the start of the season. The 76ers, Raptors, and Bucks would all be waiting, and Stevens seemed to almost warn that a messy start could loom as this new group tries to find its way.
Instead, Boston has now rebounded from the opening-night loss to the 76ers by winning three games in a row, none more impressive than Wednesday’s.
“We were down  to a very good team,” Marcus Smart said. “Usually when they’ve got a lead like that, they keep it. It was just one of those nights for us. We had to dig deep down inside of ourselves, trust one another, and lean on each other.”
Celtics forward Jaylen Brown sat out because of an illness, so Smart happily stepped forward into a role as the prime Giannis Antetokounmpo agitator. He took charges, he almost belly-flopped onto the reigning MVP to sell a foul after a slight tug, he bodied him in the paint, 6 feet 3 inches of fury against 6 feet 11 inches of power.
Antetokounmpo finished with 22 points and 14 rebounds, but he never seized control of the game the way often does.
“Every time I’m boxing him out, he’s trying to throw me out of the way,” Smart said. “It lets me know he’s frustrated, I’m getting to him, especially when he’s not getting to the ball, or he’s not getting to the rim, or he’s not getting the shots that he usually gets.”
Kemba Walker had 32 points, 6 rebounds, and 6 assists for the Celtics, Tatum added 25 points, and when Smart wasn’t tussling with Antetokounmpo, he was drilling five 3-pointers en route to 19 points.
With Brown and center Enes Kanter (knee) out, Stevens leaned more heavily on his starters than he probably wanted to this early. Walker, Smart, Tatum, and Gordon Hayward all played at least 37 minutes, but that seemed like the only real way to craft this comeback after a sluggish first half.
“We could have easily lost this game by a lot,” Walker said. “But we kept on talking to each other, just keeping each other confident, and that’s going to be important for us throughout this year.”
The Celtics trailed by 16 points at the break, but Stevens was encouraged by his players’ approach in the locker room. They were not sulking or pointing fingers or shrugging; they were trying to figure out how to make this right.
They said they had to move the ball with more urgency on offense, and they had to make sure that if they did miss shots, it did not affect their defense. Then just 51 seconds into the third quarter, Hayward and Tatum had drilled back-to-back 3-pointers.
Before long the vibe shifted, the crowd woke up, and Pierce started getting antsy in his courtside seat.
With Milwaukee leading by 13 midway through the third quarter, the Celtics unspooled a 33-9 run that flipped that big deficit into an 81-70 lead. And contributions came from all corners.
“It’s a lot of fun playing like that and we have too many weapons not to play like that,” Hayward said. “Too many guys that are so talented, so when we move the ball side to side, we are really tough to guard. Everyone gets going, the energy picks up, that’s when it’s really fun.”
Hayward, who’d appeared tentative in the first half, began to carve through the paint and drill his short pull-ups. Smart began to really irk Antetokounmpo. Walker hit some 3-pointers and drew some fouls and began to shimmy like he does when everything is going just right. In the second half he attempted 11 of his 15 free throws and scored 21 of his 32 points.
“Whenever we got stops, we got the rebound, we pushed the basketball, and made the right play,” Walker said. “We have a bunch of unselfish guys, and it was an unbelievable comeback.”
In the second half the Celtics made 54.2 percent of their shots, 52.4 percent of their 3-pointers, and 100 percent of their free throws, as they outscored the Bucks, 74-47. Milwaukee, which started the game by making 5 of 9 3-pointers, connected on just 5 of 23 after the break.