BOSTON — What, you thought it was going to be easy?

Not these Knicks. They don’t do simple.

Even without injured superstar Jayson Tatum, the Celtics weren’t going to roll over, and the Knicks didn’t bring the requisite effort and execution at either end of the floor to end the defending champions’ season on their home floor Wednesday.

Their 3-point and transition defense was shaky. The offense was too isolation-heavy and stagnant. And, in a reversal of this series, the Knicks couldn’t respond to a second-half run.

So Tom Thibodeau’s team will have to wait at least another 48 hours to advance to the franchise’s first Eastern Conference finals in 25 years after this 127-102 beatdown at TD Garden.

Game 6 is Friday night at Madison Square Garden, when the Knicks will have another shot to close out a playoff series at home for the first time since 1999.

“They’re a really good team, even without [Tatum],” Mikal Bridges said after the Knicks lost for the first time on the road this postseason in six tries. “We just didn’t bring that fight, we didn’t talk to each other defensively and [we] gave everybody confidence.”

The Celtics buried the Knicks from deep, hitting 22 3-pointers on 49 attempts to extend the series. Jaylen Brown and Derrick White hit 10 of them, and the Knicks were outrebounded by four and outscored in the paint by 10.

Boston blew the game open in a third quarter in which it took more free throws (18) than the Knicks had points (17).

“Nothing to do with the refs,” Bridges said. “We just didn’t bring it defensively. Refs can’t control how hard we play.”

The best player on the floor on this night, Brown took over in the quarter and finished with 26 points, 12 assists and eight rebounds. White tallied 34 and Payton Pritchard added 17 off the bench.

Asked what disappointed him the most about this effort, Jalen Brunson said, “A lot,” but declined to offer any details. He offered a similar non-answer about the ugly third quarter.

“They came out with a sense of urgency that we need,” he said.

It was a second-half implosion by the Knicks, who were outscored by 34 points over the final 32:30 after building a nine-point lead early in the second quarter. Brunson (team-high 22 points) was held to nine points following halftime before fouling out.

Bridges and OG Anunoby shot a combined 5-of-26 from the field and Karl-Anthony Towns was outplayed for large stretches by former Knick Luke Kornet. Josh Hart (24 points, seven rebounds) and Mitchell Robinson (eight points, 13 rebounds) were the lone bright spots in this forgettable performance.

“The start of the second quarter, we didn’t play well,” Thibodeau said. “We had a lead. Didn’t play tough with the lead. Came out to start the third, didn’t play well there. Can’t afford to do that.”

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For the first time in the series, the Knicks were ahead after 12 minutes. It was even at halftime. Then, everything went wrong.

The Celtics started the third quarter fast, building a seven-point lead after White’s seventh 3-pointer of the night. Boston scored on its first eight possessions of the quarter and took 15 free throws over the first 6:13.

After a Hart 3-pointer gave the Knicks a three-point lead, Boston scored 22 of the game’s next 28 points, creating a 13-point cushion. It was a brutal third quarter for the Knicks, who were held without a made field goal over the final 4:25 and trailed by 15 entering the fourth.

“Third quarter was terrible — offensively, defensively,” Hart said. “Watch film and figure it out.”

No comeback was coming.

The lead kept growing, up to 28 at one point. Either this was the start of the shorthanded Celtics rallying from a huge 3-1 series deficit or one last hurrah for the reigning champs in front of their home crowd.

The Knicks and their fans can only hope it’s the latter.

“We just gotta play desperate. I don’t think we did that,” Bridges said. “We gotta come out aggressive throughout the whole game.”