DALLAS — Xavier Tillman played as many minutes as everyone else who sat inside TD Garden last week watching the Celtics beat the Mavericks in Games 1 and 2 of the NBA Finals.

But on Wednesday, in a thrilling Game 3 win, he was at the center of the action.

Tillman was crucial to both Boston’s win and plan to replace an injured Kristaps Porzingis, who was ruled out pregame due to a tendon dislocation in his left ankle. While Porzingis sat, Tillman played 11 minutes, recording three points, four rebounds and a couple blocks. Defensively, he switched onto Dallas star Luka Doncic, whom he stuffed him once at the rim, and even danced with Kyrie Irving.

He was everything and everywhere Boston needed him to be.

“We needed somebody from the bench to come in and give us minutes, and Xavier was amazing tonight,” Al Horford said after the 106-99 win. “His energy, defensively he held his own, time and time again. He was just ready for the moment.”

Jaylen Brown will remember Tillman’s performance for one shot: a 3-pointer he canned late in the third quarter, as Boston built toward a 21-point lead. Brown drove and kicked the ball to Tillman, wide open in the corner. Splash.

The Celtics’ bench went wild.

“I just knew it was going in. I don’t know how to tell you. It was just like a divine experience,” Brown said. “I knew X was going to make that shot, and he did. That was big for us. I thought X was great.”

Tillman, an ex-Grizzlie, joined Boston at February’s trade deadline. The Celtics acquired him in exchange for Lamar Stevens and a couple second-round picks, thinking of a game like Wednesday’s when they might need a versatile backup. Porzingis, who missed 25 regular-season games due to injury and rest, was out again in Game 3.

Without their best rim protector, Boston knew the Mavericks would try to attack downhill, so they opted for a switching scheme that would cut drives off early instead of discouraging them late. Tillman, at 6-foot-8 and 245 pounds, occasionally played center, and routinely met the moment.

“I feel like switching is like my bread-and-butter defensively,” he said. “That’s something that allows me to use my quick feet and my strength to be able to bump guys, versus being in drop coverage and having to use my vertical leap. So for me being able to switch and stuff like that, is more catered for me.”

Horford emphasized Tillman’s defense multiple times post-game.

“He came in and he knocked down that big three as well. But defensively he was special,” Horford said. “I’m just proud of him because he didn’t get an opportunity the first two games, obviously, and he’s stayed with it, really took advantage of it. These are the type of games people remember forever, the type of impact he had.”

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Celtics coach Joe Mazzulla explained his confidence in Tillman by referencing his prior playoff experience in Memphis. Tillman, 25, often executed a switching scheme with the Grizzlies, and from year to year, toggled between regular starter and backup who gathered dust at the end of the bench.

“He played in the Western Conference for three or four years, and so he’s played against the Mavs. And so he’s had that experience,” Mazzulla said. “Memphis has used a similar game plan, so he was used to that. I thought with that comfort level of playing against those guys for so long, he was going to be able to execute the things that we needed to execute.”

It’s unknown if Tillman will be needed Friday in Game 4. But based on Wednesday’s results, it’s safe to say he’ll be prepared.

“This is something that I’ve been preparing for, I guess, for a while now,” he said. “So I was ready for the opportunity.”