Kristaps Porzingis’ troublesome left Achilles flared up again and kept him out of Thursday night’s game against the Bulls. There is concern that it could be a lingering issue that forces him to be in and out of the lineup.

Porzingis, who recently missed three games because of the Achilles, texted the Knicks during the day to tell them he was feeling sore. He tested it before the game and Jeff Hornacek said the training staff didn’t like what it saw.

As of now, Hornacek said the Knicks aren’t thinking about shutting down Porzingis for a little while to let it calm down. They already did that. They will closely monitor him, though.

“It flared up again,” Hornacek said. “So the trainers will continue to look at it. Somehow if it ends up being off and on, that’s not great for us. If he comes out there and says, ‘I can play tonight,’ we’re not going to say no necessarily. We got to trust the player.”

Porzingis has been struggling with his shot lately. He was 6-for-23 in the previous two games and air-balled a three-pointer in the closing seconds Wednesday night, leading to the fast-break basket at the buzzer by the 76ers that beat the Knicks. Hornacek believes it’s all tied to the Achilles.

“You could just tell with his explosion on his legs on his shot, it was all arm,” Hornacek said. “So maybe he’s trying to favor it. It’s tight and stiff and sore. We try and leave it up to him and see if he can play and go through it. But he’s not able to.’’

Porzingis will get a few days to rest his Achilles because the Knicks don’t play again until Sunday. But that’s the start of a back-to-back and a stretch of four games in five nights.

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The prudent thing might be to give him an extended breather, but Hornacek said that hasn’t been discussed yet, and that there’s no fear that it could worsen.

“No one has come to us and said, ‘Hey, we need to not sit him this long,’ ” Hornacek said. “None of that’s been said.

“We rely on the trainers for that stuff. Their thoughts are it’s sore and aches, but it’s not like there’s a little tear in there and he can tear it worse. It’s just a matter of the stiffness, probably more of a strain. They don’t think it’s a concern, if he says he can play and he feels good, that he is going to make it worse.”

Hornacek also said Joakim Noah, who had left shoulder surgery that ended his 2015-16 season, is playing through a sore right shoulder.

“He’s had it looked at. It’s fine,’’ he said. “There’s just some little stuff in there. When he lifts his arm in one certain way, he feels it catch and sometimes it’s on rebounds. He’s battling through it.”

Melo slipping

Carmelo Anthony, an All-Star starter in each of his five full seasons as a Knick, has some ground to make up if he’s going to make it six straight.

Anthony dropped from fifth to sixth among Eastern Conference frontcourt players in the second returns of the fan balloting. Anthony trails the third starter by more than 145,000 votes. Porzingis is seventh, roughly 3,600 votes behind Anthony.

LeBron James, Giannis Antetokounmpo and Kevin Love are the East’s leading frontcourt vote-getters.

Fans will account for 50 percent of the vote and the players and media 25 percent each. Voting ends Monday, and the starters will be announced Thursday. The coaches will pick the reserves, who will be revealed Jan. 26.