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 did not participate in the second half of the Knicks’ comeback victory in Game 1 and struggled during his 13 minutes on the court in the first half, failing to score any points.

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

According to coach Joe Mazzulla, Porzingis has been dealing with an ongoing issue since his return, trying to push through and manage it as best as he can. Mazzulla mentioned on Tuesday that the problem became too much for Porzingis during the game, despite his efforts to work through it since coming back. While Porzingis has made himself available and put in great effort, it was challenging for him to continue playing on Monday.

After Porzingis joined the Celtics, he had been a dominant force against the Knicks, averaging 22.4 points, 5.7 rebounds, and 1.7 blocks per game. He also had an impressive 50 percent success rate from the 3-point range in seven regular-season games.

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