Kristaps Porzingis’ absence could prove costly for the Celtics if his injury lingers deep into the NBA postseason.

But so far, Boston hasn’t missed its unicorn big man.

The Celtics demolished the Heat in their first game after Porzingis’ calf strain, then followed that up with a 25-point laugher in Game 1 of the Eastern Conference semifinals, walloping the Cavaliers 120-95 Tuesday night at TD Garden.

Porzingis can thank a strong shift off the bench from backup center Luke Kornet, who delivered what head coach Joe Mazzulla called a “tremendous” performance.

Al Horford’s understudy can’t come close to replicating Porzingis’ offensive talent, but he was a difference-maker defensively against the Cavs. Kornet grabbed 10 rebounds — including six off the offensive glass — and blocked two shots in the blowout win.

“I thought Luke’s presence was big for us tonight,” Mazzulla said.

The Celtics owned a decisive edge in rebounding (55-38) against a Cleveland team that was playing without injured center Jarrett Allen. Horford finished with eight boards, and Jayson Tatum had 11.

Kornet also forced misses by Evan Mobley and Donovan Mitchell with strong contests and even delivered a few offensive highlights. He slammed home a loud dunk late in the third quarter and had a nifty feed to Jayson Tatum for one of his two assists.

Twice, Kornet received a rousing ovation from the Garden crowd when Mazzulla subbed him out. He set a playoff career high with 21 minutes played.

“I thought Luke was exceptional when he came in,” said Jaylen Brown, who scored a team-high 29 points for Boston. “He was great. He came in, he added value to the second group, he contested shots at the rim, he rebounded and he made timely plays on offense. He’s had some injuries up and down, but Luke has been tremendous for our team.”

Reserve wing Sam Hauser didn’t score a point but was a plus-22 in 17 minutes for Boston. Payton Pritchard shook off an ugly 1-for-6 start from 3-point range to finish with 16 points.

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“(Kornet), Payton, X, Sam — those guys had great minutes,” Mazzulla said. “Obviously, Luke started it off. … You can’t be a good team unless those guys win you games, and they’ve all played a huge part in winning games. Whether it’s four (games) in five nights, back-to-backs — doesn’t matter, they bring it. It’s important to have guys like that.”

Initial reports indicated Porzingis was unlikely to return before the conference finals at the earliest. Mazzulla would not say Tuesday whether that timeline has changed.

“I have no idea,” the Celtics’ coach said. “I just know he’s working every day to get back as fast as he can. He’s involved in all the meetings, shoot-arounds, drills and then doing what he can treatment-wise.”

Mazzulla said earlier Tuesday on 98.5 The Sports Hub that Porzingis’ injury was “definitely better than we thought.” Porzingis offered a similar assessment on Sunday, but said he was “not super close” to returning.