BOSTON — The Knicks seemed to think that all they needed to do to move on to the Eastern Conference finals was to show up.

And maybe they were right, but we’ll never know because too many of them didn’t.

Jalen Brunson played with the fire he always does before fouling out. Josh Hart actually bled for it. But Karl-Anthony Towns was outplayed by Luke Kornet. OG Anunoby endured a nightmarish offensive performance. And the same mistakes that the Knicks had made to fall behind by huge deficits regularly in the series surfaced again — open shooters lost in transition, poor communication as if they’d just met outside TD Garden.

The result was a 127-102 demolition as Boston forced a Game 6 back at Madison Square Garden on Friday night.

And if there was one thing the Knicks did count on and did deliver was a knowledge that just because the Celtics were without Jayson Tatum, who suffered a devastating ruptured Achilles tendon Monday in Game 4, they were not going to be an easy mark.

It was the Celtics who took this game over in the second half, fighting off elimination and cutting the Knicks’ series lead to three games to two.

The Knicks lost a chance to give themselves four days off to prepare for what they hoped would be their first trip to the conference finals since 2000.

“I mean, we were up 3-1 and this game could have ended their season,” Hart said. “So obviously they were going to come out with unbelievable intensity and energy no matter who’s playing. So I don’t think that’s any added thing. I think they came out aggressive. Obviously they don’t want their season to end. They’re the defending champs.”

The frustration was evident and understandably so, but much of it their own doing. Brunson, who had no fouls at halftime, was whistled for five in the third quarter and then fouled out with 7:19 left — the Knicks attempting a challenge that failed as he finally left the floor after arguing with referee Tony Brothers.

Brunson finished with 22 points. Hart led the Knicks with 24 points and Towns finished with 19 points and eight rebounds, although much of it came in garbage time before Knicks coach Tom Thibodeau cleared the bench with 2:34 to play. Anunoby was 1-for-12 shooting. Mitchell Robinson did deliver 13 rebounds and was a surprising 6-for-6 from the free throw line, forcing Boston to abandon hacking him.

But the Knicks pointed to the defensive end for where the troubles began — communication an issue again as shooters were left open repeatedly.

Derrick White scored 34 points to make up for the scoring loss of Tatum. Maybe even more important were the seven blocks, 10 points and nine rebounds Kornet provided off the bench. With Kristaps Porzingis starting but playing just 12 ineffective minutes, Kornet started the second half and was a force in the paint.

The Knicks led by nine points in the first half but the Celtics fought back to tie it at 59 at halftime. Boston pummeled the Knicks in the third quarter, outscoring them 32-17 and stretching its lead to 27 points in the second half to ensure there would be no comebacks like the first two games.

When the game turned in the third quarter, the Knicks were visibly frustrated. Late in the first half Jaylen Brown and Hart were shoving each other on every screen and just 19 seconds into the third quarter Brown took a running start and delivered a shoulder to Hart and when they got tangled, Hart shoved him away and the two then had to be separated — each assessed a technical foul.

Hart kept his cool and was one of the few players showing fight at all. But the officials whistled the Knicks for 10 fouls in the quarter, sending Boston to the line 18 times after they shot just eight in the entire first half.

“I knew the type of aggression that would be involved in this game,” Thibodeau said. “So I think we’ve got to understand that and it’s a battle on every possession.”

“It’s just who they are as people, the character of the guys,” Boston coach Joe Mazzulla said. “ . . . At the end of the day, you don’t get to pick out the test that you have. You just pick how you respond to them. That’s how life works.”

If the Knicks thought it wasn’t going to be easy, the evidence came quickly. In the first half, Hart was bleeding badly after taking an inadvertent elbow above his left eye, Towns was limited to just 11 minutes with three fouls and White was doing what he has done from beyond the arc repeatedly.

“Yes, they are missing a big piece,” Brunson said. “But like I said before, they are a team, a well-oiled machine that has been situations where they have played without him. And so, they’ve played well and we need to understand that and trust the game plan and play to win. It’s that simple.”

They didn’t. Now the Knicks will have to take that approach Friday at the Garden.