A miscue by the Celtics’ best player doomed Boston in the closing seconds of its Game 2 loss.

One day after the New York Knicks stunned their Atlantic Division rivals 91-90 to take a 2-0 lead in the Eastern Conference semifinals, Jayson Tatum shouldered the blame for the Celtics’ ugly final possession. With Boston down one, he dribbled diagonally into traffic and was unable to get a shot off while being swarmed by Mikal Bridges and OG Anunoby.

“I just kind of made the wrong read,” Tatum said Thursday. “Went into a crowded area. Obviously, if I had that opportunity again, or the next time it presented itself, just have a different outcome. But I take responsibility for us not at least getting a shot up at the end of the game.”

It was a discouraging end to another underwhelming performance by Boston’s centerpiece superstar, who struggled in Games 1 and 2 against New York after dominating the Orlando Magic in the opening round of the NBA playoffs. Tatum went 12-for-42 from the field and 5-for-20 from 3-point range over the two losses, both of which featured blown 20-point second-half leads by the defending champion Celtics.

“I take full ownership of the way that I’ve played in this series,” Tatum said. “And I can’t sugarcoat anything — I need to be better. I expect to be a lot better.”

He isn’t the only Celtics shooter who’s scuffling.

Boston’s field-goal percentages in Games 1 and 2 (35.1% and 36.2%, respectively) were its two worst of the season. The Celtics went 25-for-100 on 3-point attempts and were especially erratic during New York’s late-game comebacks. They were 4-for-21 (4-for-15 from three) in the fourth quarter of Game 1 and 5-for-24 (2-for-11) in the fourth in Game 2.

And that’s despite the vast majority of their long-range attempts coming on clean looks. Seventy-five of their 3-pointers in the series were classified as “open” or “wide-open” by NBA player tracking, meaning there was not a defender within four feet of the shooter.

Though the Celtics have faced criticism for their shot selection, Tatum said their primary problem has been execution. He doesn’t believe the team should alter its approach.

“We have a way that we play,” Tatum said. “We have an identity, and it can’t waver just because we missing shots. We can’t change who we are. And I always say, you’ve got to be the same person when things are going well and when things aren’t going well. And it’s not about our season being two games from over. Like, that hasn’t crossed any of our minds. We got a game on Saturday at 3:30 that should be a lot of fun. If you’re a competitor, you should really look forward to that moment. We put ourselves in this position, and we got a game on Saturday that we’re all looking forward to.”

Celtics head coach Joe Mazzulla expects a bounce-back performance from his top scorer as the series shifts to Madison Square Garden for a must-win Game 3.

“I appreciate the fact that he would (take responsibility), but I think it’s on everybody,” Mazzulla said. “We all could be better. I could be better in the things that I can help our team execute on. Our entire team can be better, and yeah, he can, too. It just says who he is, taking responsibility for that, and I expect him to be better. There’s no doubt in my mind.”

Porzingis fighting illness

When Kristaps Porzingis concluded his first six-minute shift Wednesday night, he looked like he’d just run a marathon.

His breathing was heavy. He was visibly fatigued. When he took his seat on the Celtics bench, he dropped his head and stared at the TD Garden parquet.

The Boston big man was active and available for what teammate Payton Pritchard called a “must-win” Game 2 for his squad, trying to play through an illness that’s hindered him for months. But he clearly was not himself.

Coming off the bench for just the fifth time in his NBA career, Porzingis was limited to 14 minutes and finished as a team-worst minus-9. Two nights earlier, he exited midway through the second quarter and did not return as the Celtics lost 108-105 in overtime.

“I’m dealing with some, I don’t know how to call it, but I’m just not feeling my best,” Porzingis said after Game 2. “I’m not feeling my best at all, but it just kills me inside that it’s happening in this moment, and what I’m super appreciative about is about the support that I have on the inside, and also probably from the fans. But especially here inside the organization in this tough moment for me to not be able to be with the guys. It’s not no injury or nothing, but I’m just not feeling my best, and it’s tough for me, honestly. But who cares? Nobody feels sorry for us, sorry for me, and we have to keep going.”

Porzingis said his current condition is “probably” related to the viral illness that sidelined him for nine games in late February and early March. Mazzulla said the same on Tuesday, acknowledging that the 7-foot-3 center has been “kind of dealing with it on and off, fighting through it, working through it, doing the best that he can” since he returned to the lineup on March 15.

“I’ve had, like, ups and downs throughout up until this point,” Porzingis said. “Just now, I had a big crash now, and my energy, my everything, hasn’t been good. But who cares? I have to look forward, and it’ll get better from this point on.”

The Celtics certainly hope so. They were able to weather Porzingis’ extended absence last postseason — he was inactive for 11 games due to lower leg injuries, and Boston won 10 of them — but his limitations have hurt them in this series.

Porzingis has played just 27 minutes over the two games and less than four in the fourth quarter or overtime. Mazzulla kept him on the bench for the final 8:27 on Wednesday as the Knicks completed their comeback.

“You’d rather have him, and you feel his absence because of his versatility and what he’s able to do on both ends of the floor,” Mazzulla said Thursday. “Our depth is a strength of what we have. At the same time, to have a guy like KP, you want to have him as much as you can. And so we’ll look at KP to try to do that, and I know he’s doing what he can to make sure he’s available.”

Even when he has been on the court, Porzingis’ contributions have been muted. He’s scored fewer than 10 points in five of Boston’s seven playoff games after doing so just once in 42 regular-season appearances. His rebounding and block numbers are down, and his 34.9% field-goal percentage and 16.7% 3-point percentage both rank last among Celtics rotation players, though he did go 3-for-5 and 1-for-1 from three in Game 2.

Porzingis shot 41.2% from deep during the regular season, the second-best mark on the team behind bench sharpshooter Sam Hauser, who’s also struggled this postseason and missed Wednesday’s loss with an ankle sprain.

The Celtics will have two days off before Game 3 on Saturday at MSG.

“I thought those minutes (in Game 2) were really impactful for us, and he gave us everything he had,” Mazzulla said. “With these two days off, hopefully he’ll recover a little bit more. But he’s going to continue to just give us everything he has, and every game will be situational depending on how much he can go. But I’m just grateful for what KP was able to do for us last game.”