BOSTON — Joe Mazzulla walked out to midcourt in a hurry Sunday afternoon. After signaling for a timeout, he blasted one of the referees, pointing to different spots on the court to emphasize his message. The Celtics coach almost always huddles with a group of his assistants before addressing the players, but skipped that step this time, spinning straight for the bench after concluding his brief chat with the official.

With veins popping out of his neck, Mazzulla addressed the players next. Three minutes into the second quarter of Game 7, the flames of ruthless competitiveness lit up his face. With a five-point deficit already and James Harden stepping to the free-throw line for two shots, the most important moment of the season sat waiting on the other side of the timeout. Mazzulla called for the Celtics to seize it.

“We have to shift the momentum,” Mazzulla said. “We have to get it back.”

It would be easy to remember the Celtics’ 112-88 win as a laugher, a blowout, a TD Garden celebration. It became those things in the second half as Jayson Tatum, who poured in 51 points, and raced toward the highest-scoring Game 7 in NBA history. His special outing helped set up an Eastern Conference Finals rematch with the Miami Heat. But before all that, Boston needed to break through the tension of a slow start.