Kristaps Porzingis’ return from injury last week immediately improved the Celtics’ struggling rim defense.

But Boston didn’t need its unicorn big man to effectively protect the rim in Monday night’s 108-89 rout of the Miami Heat at TD Garden.

With Porzingis and primary backup Al Horford both sitting on the second night of a back-to-back, third-string center Luke Kornet enjoyed one of the best shot-blocking performances of his NBA career, tying a personal best with six rejections in the blowout win.

The last time Kornet had six blocks in a game was in April 2019, when he was playing for the New York Knicks.

Kornet also blocked three shots in just eight minutes in Sunday’s loss to Cleveland. Nearly half of his 19 blocks this season have come in the last two games.

“When he plays with a level of physicality on both ends of the floor, it makes our team different,” Celtics coach Joe Mazzulla said. “And I think the two things — his two biggest strengths — are his physicality and rim protection when he’s at his best, and then his screening versus different coverages. And I thought we saw both of those throughout the game, and he’s just got to continue to do that. But when he plays at a high level, we’re a different team, and we can go to different things.”

Supplying that physicality has been a point of emphasis for Kornet, whose winding NBA journey has spanned five franchises and included multiple G League stints. The 29-year-old said he was playing too passively at the start of this season, limiting his effectiveness.

“I was just playing, I think, too conservative in terms of not trying to foul, and I think I was kind of overdoing it in that respect,” Kornet said. “(I’m) just trying to do a better job of really meeting guys and kind of seeing what that line of physicality is. I think I was just too conservative for a lot of the early part of this year. I feel like it kind of just starts with that, and being able to be that presence for our team is something that I think is just important. I feel like we need that game to game, so just trying to fill that role.”

On Monday, Kornet helped hold the Heat to 61.1% shooting inside the restricted area and 28.6% in the rest of the paint, both well below the season averages for Celtics opponents. Olympian Bam Adebayo missed 11 of his first 14 shots for Miami.

Kornet won’t see extensive playing time in games when Porzingis and Horford are active. But with Boston choosing to limit both of their workloads for precautionary purposes, depth bigs like Kornet and Neemias Queta (two blocks Monday night) will continue to have important roles in Mazzulla’s rotation.

“Luke has just been the ultimate teammate since he’s been here,” Jaylen Brown said. “Whatever you ask him to do, he’s willing to do. He does his job each and every night. He knows the game well. Offensively, he’s just a great connector for us. And defensively, he uses his size well. So (Monday night) was an example of that. He was able to get a career high in blocks.

“When Luke’s playing well, I think we’re an even better team.”