Jayson Tatum couldn’t stop himself.

Frustrated by the officiating all night – and maybe another ugly shooting performance – the Celtics were trailing by double digits late in the fourth when Tatum was hit on the arm by Julius Randle while attempting a 3-pointer. He jumped a few times and clapped in protest. And after Josh Hart drilled a corner 3 to put the Knicks up 14, Tatum continued his complaining.

Moments later, the Celtics star headed to the showers at Madison Square Garden early, with Knicks fans, and even Hart, waving goodbye to him as he walked through the tunnel.

Tatum’s first career ejection capped a frustrating night for both he and the Celtics, who never had it going from the opening tip. Two nights after they erased a double-digit second half lead in a wild win over the 76ers, the C’s never sustained any momentum, they missed Jaylen Brown (out for personal reasons) and they couldn’t overcome one of their worst outside shooting displays of the season in a 109-94 loss to the Knicks.

The last time they visited MSG, on Nov. 5, the Celtics canned a franchise record 27 3-pointers. In their return on Monday night, they drifted back to the other extreme as they finished 9-for-42 from downtown – which included 15 consecutive misses during one stretch in the first half – for a 21.4% performance that was their worst of the season.

“I just didn’t think it was our night,” Celtics coach Joe Mazzulla told reporters in New York.

Randle and Immanuel Quickley each scored 23 points for the Knicks, who won their sixth consecutive game and second of three meetings this season with the Celtics, suddenly looking like a legitimate challenger in a potential playoff series. The Celtics, though tied with the Bucks in the standings, fell into second place in the Eastern Conference by percentage points.

Following his historic All-Star Game, Tatum has struggled in each of the Celtics’ first three games coming out of the break. The C’s overcame it the first two games, with Tatum saving his poor performances with winning shots, including the gamewinner on Saturday in Philadelphia. But without Brown, Boston needed a better night from Tatum. His slump continued.

Tatum finished with 14 points on 6-for-18 shooting, including 1-for-9 from long range. After getting just two free-throw attempts on Saturday, he only went to the line once on Monday – and it was a technical free throw.

His frustrations started to boil over late in the third quarter. Tatum threw down a dunk with 44 seconds remaining in the period – which cut the Celtics’ deficit to 14 – but when he landed back on the court, he took issue with an official, thinking he was pushed by a defender. He gestured with his hands a few times to an official and was assessed a technical foul.

The Celtics were well on their way to a loss in the fourth when Tatum was tossed.

“The first tech came from the reaction from the dunk,” Tatum told reporters. “That one was tough, because they literally guided me to the rim. two hands on my back, that would have been scary, right? … I was upset about the three, I felt like he smacked my arm. It didn’t go my way.”

What triggered the ejection?

“I just told him, this is probably the best officiated game I’ve been a part of,” Tatum said, clearly in a sarcastic manner. “It didn’t go over so well.”

Tatum’s is shooting 36.6% and 24.1% from 3-point range since returning from the break. It’s been nearly two weeks since he was home in Boston, and the Celtics’ return this week should be warmly welcomed.

Al Horford isn’t concerned about him. “It’s a long season,” Horford told reporters. “Jayson’s been playing, as we all know, at an MVP level for most of the season. I mean, if he can go all 82 playing like that, that’s just tough. There’s going to be ups and downs, and he’s the last guy I’m worried about. I know what he’s about, and my biggest thing with him is making sure he feels right, that he’s good to go in April once the playoffs start.

“I’m not worried about it. Through the season, you’re going to have an off game here and there, and that’s just a part of it.”

The Celtics played from behind seemingly the entire night because of their shooting woes. They scored 15 points in the first quarter – a season-low – and fell behind by 20 in the second quarter. They responded with a 12-0 run fueled by strong defense and attacking the basket, but it was just a tease. They cut their deficit to nine in the third quarter, but the Knicks had an answer every time they threatened.