Jaylen Brown could barely muster any reaction.

He took a long pause, thought, and still could not find the right words. 

“It’s tough,” Brown said. “There’s really not a lot to say.” 

His longtime partner in crime is likely out for the long term. 

The Celtics lost much more than just the game Monday night.

They lost their best player in Jayson Tatum and much of any hope to come back in the series. 

Depending on the severity, even beyond this series, they could lose him for all of next season, too. 

Tatum crumbled to the ground with a noncontact injury while going for a loose ball late in the fourth quarter of the Knicks’ 121-113 Game 4 win over the Celtics on Monday night at Madison Square Garden.

He immediately grabbed at his lower right leg and rolled around in pain. 

He was helped off the court and moments later was in a wheelchair.

An emotional Tatum held his head in his hands as he was rolled away. 

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Knicks players were unanimous in sending their prayers to Tatum.

They stood and clapped as Tatum was helped off the court in front of their bench. 

Celtics coach Joe Mazzulla did not have an update after the game, other than calling it a lower-body injury and saying Tatum will have an MRI on Tuesday.

But it appeared to be an Achilles injury. 

One could hear a pin drop in the Celtics locker room after the game.

The loss almost became secondary to Tatum’s injury. 

“The loss is the loss. More importantly, it’s just Tatum that I’m worried about,” Al Horford said. “Just making sure I’m here for him. That’s my priority. … It’s very concerning, just from the care that I have for him and what he means to us, what he means to Boston. It’s just very tough for us and more importantly very tough for him right now.” 

The Knicks led 111-104 at the time of the injury. 

“This is a big hit for us,” Kristaps Porzingis said. “100 percent.” 

Tatum had been enjoying by far his best game of the series, erupting for 42 points on 16-for-28 shooting from the field and hitting 7 of 16 from 3-point range.

He also had eight rebounds, four assists, four steals and two blocks. 

The Knicks were unable to stop him.

The only thing that could was the injury. 

He appeared to suffer the injury while planting with his right foot and attempting to corral the ball after Brown had lost it. 

“Obviously, you’re always worried about someone’s health,” Mazzulla said. “The fact that he had to be carried off, he’s the type of guy, he [usually] gets right up. He didn’t. We’ll know [Tuesday] exactly what it is, but it’s tough to watch a guy like him get carried off.”