Kristaps Porzingis has yet to shake the illness that sidelined him for nine games late in the regular season.

And now, it’s affecting his postseason availability.

The Celtics big man sat out the final 2 1/2 quarters of Monday night’s 108-105 overtime loss to the New York Knicks, with the team announcing he was questionable to return with an illness. Porzingis warmed up after halftime but spent the entire second half in the locker room as Boston blew a 20-point lead in Game 1 of the Eastern Conference semifinals.

After initially saying Tuesday that he had “no idea” whether Porzingis’ ailment was related to the one he dealt with in late February and early March, Celtics head coach Joe Mazzulla acknowledged Porzingis has been feeling the effects of that “viral illness” since he returned to action on March 15.

“Since he came back, I think he’s been kind of dealing with it on and off, fighting through it, working through it, doing the best that he can,” Mazzulla said. “I think it was just too much for him in that game. I didn’t know that was going to happen until I found out about it in that game. But he had been working through it since he got back, and he’s done a great job of being available. It was just tough for him to continue (Monday), so we’ll see kind of how he handles that.”

Mazzulla and multiple Boston players admitted losing Porzingis hurt the Celtics in Game 1, as they lacked his rare blend of rim protection and perimeter shooting ability and were limited in which lineup options they could utilize, especially after backup center Luke Kornet picked up his fifth foul with 8:47 left in regulation.

“Yes, I feel like Kristaps being out definitely impacted our group,” said veteran big Al Horford, who played 24 minutes in the second half and overtime. “But ultimately, I give the Knicks credit. They were the better team (Monday).”

After missing 40 games during the regular season, Porzingis played in all five of the Celtics’ first-round series against Orlando. But his production was erratic. He had 20 points and 10 rebounds in Game 2 and 19 points in 22 minutes in Game 4, but he totaled 21 points over the other three contests and shot the ball poorly throughout, making 35.2% of his field goals and 11.8% of his 3-pointers.

Porzingis played 13 minutes Monday night and finished with zero points and four rebounds on 0-for-4 shooting. The Celtics officially listed him as probable for Game 2 on Wednesday, but his workload will be worth monitoring if he is able to go.

“He’s a little bit better,” Mazzulla said. “We’ll see how he feels. Just kind of day to day.”

The Celtics also lost rotation sharpshooter Sam Hauser to an ankle sprain during Monday’s loss. He’s also considered “day to day,” per Mazzulla, but likely will not suit up for Game 2, as he was listed as doubtful on Boston’s latest injury report.

“We’ll see how that goes,” Mazzulla said.

Hauser and Porzingis ranked first and second among Celtics players in 3-point shooting percentage this season. The former also has been inconsistent so far this postseason, however (zero points in the first three games against the Magic), and didn’t enter Monday’s game until the third quarter. He suffered his injury four minutes later after missing two threes and posting a minus-7 in limited action.

It was the first time in 175 appearances over the last two seasons that Hauser did not see the floor in the first half.

“Everything was just about the matchups and the sub patterns,” Mazzulla said. “Everything got changed with foul trouble and matching certain sub patterns for them, so you work to do that throughout a game.”

Mazzulla defends 3-point parade

The theme of Mazzulla’s day-after news conference could be summed up in one sentence:

“If the open ones go in, we’re not having this conversation.”

He was referring, of course, to the 60 3-pointers his Celtics launched Monday night, a new single-game postseason record. Boston missed 45 of those — also a playoff record — as their 20-point lead vanished over the final 18 minutes.

Though he acknowledged there were “10 or 11 (shots) you could probably take back,” Mazzulla said he “loved the vast majority” of them despite the fact that just 15 found the basket.

The Celtics head coach underscored this opinion when asked about Boston’s defense against New York’s 3-point shooters. The Knicks were able to generate a number of open looks from deep during their comeback and made more than 50% of their second-half threes.

When that success rate was mentioned, a wry smile appeared on Mazzulla’s face.

“So they made them? Man, what a novel idea,” he quipped. “That’s a novel concept, huh? Just make ‘em when you’re open? Yeah.”

Pressed to answer the original question, Mazzulla added:

“It’s such a cool duality of your question to where I’m getting asked all the ones that we missed and you come in and start with, ‘Well, they shot 50% (from three) in the second half.’ And I’m like, oh man, the duality of that question is really cool to talk about. You know what I mean? So, sorry. We start the press conference off by saying, you know, you took all these and you missed ‘em. And then you say they shot 50% in the second half from three. Yeah, you’ve gotta make them. Duality at its finest.”

Fifty-six of the 60 threes Boston attempted in the loss were classified as “open” or “wide-open” by NBA player tracking. The Celtics shot 35% and 41% in those situations during the regular season, respectively. On Monday, they went 7-for-32 on open looks (22%) and 7-for-24 on wide-open looks (29%).

Hit just a few more of those, Mazzulla said, and the outcome would have been different.

Off the rim

The Celtics weren’t the only heavy favorite to open the conference semifinals with an upset loss. The top-seeded Cleveland Cavaliers and Oklahoma City Thunder also suffered stunning Game 1 defeats (to the Indiana Pacers and Denver Nuggets, respectively), with OKC blowing an 11-point lead in the final 4:08 on Monday night. … Brad Stevens received one vote for NBA Executive of the Year, the award he won last season. Thunder general manager Sam Presti topped the voting, beating out Cavs counterpart Koby Altman.