BOSTON — In March, an upper respiratory illness cost Kristaps Porzingis eight games.

It sounds as if that problem hasn’t completely gone away for the former Knick.

The 7-foot-2 center didn’t participate in the second half of the Knicks’ comeback victory in Game 1 and was unable to make an impact in his 13 scoreless minutes during the first half.

He is probable to play in Wednesday night’s Game 2 at TD Garden.

“Since his return, he’s been dealing with it intermittently. He’s been pushing through it, managing it, and giving it his all,” coach Joe Mazzulla mentioned on Tuesday. “During that particular game, it was just too overwhelming for him. … He had been persevering since his comeback. He’s done commendably well at remaining available, but continuing [on Monday] proved to be too challenging for him.”

After joining the Celtics, Porzingis had become a significant obstacle for the Knicks, averaging 22.4 points, 5.7 rebounds, and 1.7 blocks, while impressively shooting 50 percent from beyond the arc over seven regular-season matchups.

In the opening-round series against the Magic, Porzingis averaged 12.0 points and 5.8 rebounds, but shot only 11.8 percent from 3-point range.

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“Kristaps being out, I think definitely impacted our group,” backup center Al Horford said. “But ultimately, I just give the Knicks credit. They were the better team.”

It doesn’t sound as if Mitchell Robinson’s free-throw shooting woes and the Celtics intentionally fouling him is going to impact his playing time.

Coach Tom Thibodeau highlighted the 7-footer’s overall impact despite going 3-of-10 at the charity stripe.

He was a team-best plus-13 in 21 minutes.

“I think just the strengths that he brings to the team, the rebounding aspect of it, the rim protection, the ability to switch,” Thibodeau said. “His size is very important for us. And so, that’s what we’re trying to take advantage of. I’m sure he’s been fouled [intentionally] before and he’s got to just step up there and make.”

Robinson wasn’t the only Knick who struggled at the line.

Josh Hart missed three free throws and Cam Payne missed two as the team shot a dreadful 17-of-31.

The Celtics could be without reserve forward Sam Hauser, who suffered a sprained right ankle on Monday.

He is doubtful for Game 2.

— Additional reporting by Jared Schwartz