NEW YORK — The Knicks, who opened their season with a blowout loss in Boston, entered Saturday night viewing their rematch with the defending-champion Celtics as a measuring stick as they approach the All-Star break.

They exited Madison Square Garden with a clear answer about where they stand: far behind the Celtics, who routed the Knicks 131-104 just like they did in October.

The 27-point setback — New York’s largest of the season — was eye-opening. The Knicks had won seven of eight before Saturday’s game and were 1½ games behind the second-place Celtics in the Eastern Conference.

“Unacceptable,” said Knicks captain Jalen Brunson, who scored a team-high 36 points on just 18 shots. “We’re not where we want to be.”

One day earlier, he said he looked forward to seeing how his team measured up with Boston after showing improvements — and cohesion with newcomers Karl-Anthony Towns and Mikal Bridges — since the start of the campaign.

But many of the Knicks’ same problems from opening night showed up again Saturday. Specifically, they had no answer for Jayson Tatum, who threw down an enormous dunk on the second play of the contest en route to scoring 40 points. The All-Star seemed to get whatever he wanted in one-on-one matchups with New York’s wings, and shot 13 for 26 overall and 7 for 14 from the arc.

The Knicks (34-18) were without perimeter stopper OG Anunoby, who’s dealing with a foot sprain. But the Celtics were down two starters: guard Jrue Holiday and former Knick Kristaps Porzingis, who was a late scratch due to a non-COVID illness. Luke Kornet was told late in the pregame process that he’d be filling in for Porzingis. Kornet shot 7 for 7 for 14 points and added 12 boards and three blocked shots.

“Big Luke played his ass off today,” Tatum said of Kornet.

Tatum was the best player on the floor, though, and said he enjoyed putting on a show with famed actor Denzel Washington sitting courtside with friend and Academy Award-winning film director Spike Lee.

Boston (37-16) dominated most of the game, but the Knicks — powered by Brunson’s trips to the free throw line — cut their deficit to three midway through the third.

That’s when Tatum and Derrick White provided all the Celtics’ scoring during a 19-3 spurt that essentially sealed the win. Tatum sat the final six minutes of the game, and massive pockets of the crowd — including Lee and Washington — started heading for the exits several minutes early.

The Knicks — 23-6 against sub-.500 clubs, but just 11-12 against teams with records .500 or better — are trying to prove they belong with the league’s elite. They’re 0-3 against the Cleveland Cavaliers and Celtics, who sit in first and second place, respectively, in the East. And the Knicks dropped both of their contests against the Oklahoma City Thunder, who own the league’s best mark.

“We’ve had a target on our back the whole year, and we understood that going into the season. I think every game [against us] is probably a measuring stick,” White said. “It’s just on us to have the right mindset.”

In the other locker room, the Knicks said that as much as they might have improved during the season, they still aren’t close to the Celtics’ level yet.

“We’ve got a lot of work to do. Simple as that. There’s no sugarcoating it. There’s no moral wins,” said Towns, who finished with a quiet nine-point, nine-rebound outing. “If we’ve got aspirations, which I know we do, then we’ve got to find ways to win games like tonight. That’s a team that’s in the race. Great teams, championship teams, they test your discipline. And we’ve got to work on keeping our discipline for all 48 [minutes].”