A longtime Atlantic Division rival is standing in the Knicks’ way of returning to the Eastern Conference finals.

It’s just not the opponent everyone expected it to be.

There won’t be a rematch with the Celtics after all.

Instead, it’ll be the suddenly red-hot 76ers and the Knicks in the second round, starting Monday night at the Garden.

Philadelphia, the seventh seed, completed a rally from down 3-1 Saturday night, knocking off the Jayson Tatum-less Celtics at TD Garden.

These aren’t the same 76ers that went 45-37 during the regular season.

They are finally healthy.

Superstar center Joel Embiid returned after missing the first three games of the playoffs following emergency surgery for appendicitis that cost him the tail end of the regular season.

He was terrific in the series against the second-seeded Celtics, averaging 28 points and nine rebounds in four contests, although he did appear to hurt his left knee in the final seconds of Game 7.

The big three of Embiid, Paul George and Tyrese Maxey appeared in only 15 regular-season games together.

But they looked formidable against Boston.

Maxey, in particular, was brilliant.

He owned Game 7 down the stretch and finished with 30 points, 11 rebounds and seven assists.

There is recent postseason history between the two teams.

They met in a thrilling first-round series two years ago, won by the Knicks in six games.

Five of those contests were decided by single digits.

It included what the Knicks called a “dirty” play by Embiid, in which he committed a flagrant foul on Mitchell Robinson by grabbing his leg while airborne.

It led to the Knicks’ center’s ankle injury worsening, and eventually ended his postseason early.

The Knicks began preparations Saturday for their next opponent, even though they didn’t know at the time it would be Embiid and Co.

The Celtics and 76ers were hours away from meeting in Boston in a winner-take-all Game 7.

Not that it mattered much to Mike Brown.

“It’s about us, and a lot of times it’s about us even if we know who we’re playing,” the Knicks’ coach said. “There’s plenty that you can work on without knowing who your opponent is because you’re trying to get better or sharper in the areas that make you who you are.”

The Knicks will enter the next round a confident group, after eliminating the Hawks in six games.

That’s not to say that the series was easy, Brown’s team trailing 2-1 at one point.

They responded like a champion, winning the last three games by a combined 96 points.

They demolished the Hawks in the clincher, winning by a team playoff-record 51 points.

Momentum changed in a flash.

Asked if the Knicks played at a championship level in those final three games, Jalen Brunson said they performed at a level capable of getting out of the first round.

The Knicks’ response was impressive, especially considering how much the other top seeds in the Eastern Conference — the top-seeded Pistons and second-seeded Celtics — have struggled.

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Detroit needed a massive comeback Friday night from a 22-point halftime deficit against the Magic just to force a Game 7 Sunday.

The Celtics didn’t even get out of the first round.

The Knicks, meanwhile, are enjoying a weekend off.

There is a chance that when the first round is complete, they will be the highest seed left in the conference.

“I haven’t really thought about it. It’s one of those things that you can’t really worry about,” Brunson said. “You have to control what you can control. Focus on one day at a time and focus on the task at hand.”

This is now four straight seasons the Knicks have advanced past the first round of the playoffs, the first time since 1992-2000.

The series win over the Hawks was different.

For one, Brown, and not Thibodeau, was the coach.

Plus, the Knicks rallied from a series deficit, a feat last accomplished in the 2000 postseason.

It led Brown to compare this group to some elite ones he has been a part of that reached the NBA Finals, such as the Spurs, Warriors and Cavaliers.

“The first thing is they’re resilient because in the playoffs you’re going to have a lot of highs and a lot of lows, which means you’re going to face some adversity, so when [that happens], can you stay connected? And this group can,” Brown said. “So the connectedness that this group has is similar. The sacrifice that this group has is similar, the competitive spirit that this group has is similar and then the belief is similar as well.”