Celtics coach Doc Rivers was already in the locker room because of the same sequence late in the third quarter, ejected after two quick technical fouls.
When Garnett finally returned with less then 5 minutes left, Paul Pierce had just given the Celtics their first lead. And they would go on to a physical 99-92 victory Thursday night over the Mavericks.
Pierce scored 18 of his 31 points in the fourth quarter, but gave plenty of credit to his argumentative coach.
"It gave us a nice little spark," Pierce said. "It can go one of two ways. The coach is gone in the last game before the All-Star break, you can pack it in. But this group is very competitive. We're going to fight."
Boston (44-11), the defending champion, goes into the break with the most victories in the NBA. The Celtics won their ninth consecutive road game, even after trailing by as many as 15 points in the third quarter.
Pierce's tiebreaking 11-footer gave the Celtics a 93-91 lead. At the other end, he took an elbow to the head from a charging Nowitzki, who was called for the foul. Then Pierce made another step-back jumper, from 14 feet, to seal the game.
"I got in my rhythm," Pierce said. "We had a play that was working. I got into my sweet spot and took the shots."
Boston never led until Pierce made a tying 13-foot jumper while being fouled by Howard with 5:37 left, then added the free throw to make it 85-84. After Nowitzki missed two shots, stretching his quarter-long drought to six misses in a row, Pierce hit two more free throws after Josh Howard's sixth foul with 3:47 left.
"I'm going to take the blame for this. We played the Paul Pierce play different ways and had some of our best guys guarding him, but we should have double-teamed him," Mavs coach Rick Carlisle said.
Nowitzki led Dallas with 37 and Howard had 17 before fouling out.
Ray Allen, playing a night after injuring his thumb in the Celtics' victory at New Orleans, had 20 points and will play in the All-Star game still. Rajon Rondo had 19 points, 15 rebounds and 14 assists for his second career triple-double, both this season.
After Garnett was called for a non-shooting foul against Nowitzki with 2:27 left in the third quarter, they were exchanging words while setting up for the next play when official Dick Bavetta called a double technical against the two All-Stars.
Only 10 seconds later, Garnett got called for another foul -- his fifth personal -- and Rivers got a technical as Nowitzki turned around with a triumphant look on his face. After Nowitzki made the free throw and play resumed, Rivers was still arguing and got his second technical.
"Doc knew we were flat. He was trying to get us going. It was good," said assistant coach Tom Thibodeau, who took over for Rivers. "Once he got tossed, I thought that galvanized us, energized us."
Nowitzki didn't make another field goal until hitting a 3-pointer that tied the game at 89 with 1:58 left in the game. But Pierce then made two more free throws, before Nowitzki pulled up for another tying 12-footer.
Before his late spurt, Nowitzki hadn't made a field goal since 2:49 was left in the third quarter, putting the end to a 14-4 run by the Celtics.
Dallas had been up 65-50 only a few minutes earlier the altercation on Antoine Wright's 3-pointer. Wright had a steal and, while falling down, got the ball to Jason Kidd, who stepped over him on the way down the court. Kidd passed to Howard, who penetrated and then passed back to Wright, who had gotten up and was open on the left wing.
Boston started the game hitting one of its first nine shots and trailed throughout the first half. Howard's 3-pointer broke a 2-2 tie, and the Mavs continued to lead, going up 51-40 after Nowitzki scored a versatile six points in the final 1:44 of the half.
Nowitzki made an inside basket, converted two free throws after going down hard while being fouled by Rondo, then had a turnaround 17-footer.
It was drastically different from their game in Boston last month, when the Celtics built a 31-point lead before halftime and had a season-high 74 points at the break. Boston won that game 124-100.
Allen had two fouls in the first 4 1/2 minutes. He played 35 minutes a night after injuring his thumb when it got caught in the jersey of another player in New Orleans. ... Boston swept the season series against Dallas for the second straight year. Before last year, the Celtics' last sweep was 1993-94.