BOSTON — The Knicks are at a crossroads in Beantown. 

A loss in Sunday’s matinee, particularly an ugly one, would further the freshly reinforced idea they’re not equipped for the big time.

A victory would redirect the narrative and serve as evidence the Knicks aren’t going to roll over against the East’s elite. 

So is it good that the Celtics showdown arrives right after Friday’s debacle in Cleveland?

Or would it be better to have that 142-105 nastiness further in the rearview before facing another marquee opponent? 

“Ask me that after the game on Sunday,” Jalen Brunson said. 

The Knicks (37-19) have failed in most of their measuring-stick games — with important exceptions of two victories over the Nuggets — but nothing feels more relevant to their long-term success than matchups against the Celtics. 

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The two feel destined for a second-round playoff matchup, especially with the Cavaliers giving no indication they’ll concede the top seed.

In the two previous matchups this season, the Celtics boat-raced the Knicks while bombing efficient 3-pointers at will. 

The Cavaliers did much of the same in Friday night’s game that mercifully ended roughly 39 hours before tipoff at TD Garden. 

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“I love it. I feel like you want to play best at all times,” Miles McBride said of the quick turnaround. “We have to bounce back and have our heads right.” 

The most glaring problem the Knicks must address is pick-and-roll defense, specifically when it involves Karl-Anthony Towns.

Jayson Tatum punished the Knicks in the season opener by hunting Towns and ditching Mikal Bridges.

In the subsequent head-to-head two weeks ago, Tatum was unstoppable while dropping 40 points and silencing MSG. 

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One Knicks adjustment might be to instead use OG Anunoby on Tatum — Bridges was the primary defender in the first two matchups — but Anunoby just returned from injury and couldn’t slow Donovan Mitchell on Friday night. 

Towns expressed confidence the defense will rebound. 

“I don’t think that we lack a belief in doing any of that. The belief is high in this locker room. We just got to continue to work. We’ve got to continue to get better,” he said. “[Friday night] we may not have made that step forward that we talk about every day and I like to talk about every day, that we need to get that 1 percent better. We didn’t do that. But we get another chance quickly with Boston to show how resilient we are and what grit we really have to bounce back from a game like this.” 

The Knicks have to show something much greater than Friday — and even greater than the poor defensive effort in Thursday’s overtime win over the Bulls. 

Josh Hart, perhaps not coincidentally, missed those two games with a sore knee. 

“We’ve got to turn the page but we’ve got to understand where we are,” Brunson said. “We can’t just turn the page and forget about it. We’ve got to turn the page and realize what has happened [in Cleveland]. This goes back to the Chicago game. The Chicago game was not our best play, but we masked it with a win. We’ve just got to be better. Simple as that. Can’t keep pointing fingers making excuses, saying what if this, what if that, what if we had this person? No, we’ve got to go out there and play how we’re supposed to play with what we have.”