Steve Nash was concerned the Nets could be rusty after the All-Star break, and he was right. But Kyrie Irving was so good against his old team, it didn’t matter.

After putting on a show during a Christmas Day rout in Boston, Irving had an encore performance to open the second half of the season. Amid cheers of “M-V-P,” he led the Nets to a come-from-behind 121-109 win over the Celtics on Thursday before 1,374 at Barclays Center.

Irving poured in a game-high 40 points to lead the Nets (25-13) to an 11th win in their last 12 games. With Brooklyn clinging to a 108-106 lead with four minutes left, Irving had eight points in a 13-3 run to close out the game.

“That’s what makes the game beautiful is being able to have spurts. It’s just the flow of the game but when it gets to that quarter and it’s winning time I’ve always said it throughout my career I enjoy it more than anything else,” said Irving, who also had 37 in Boston on Christmas Day against his old club.

“I think any time you play your old team, there’s an added sense of whatever. Whether he says it or not, whether anybody says it or not, that’s always there I think. … It gives you a little extra juice,” said Landry Shamet.

Steve Nash agreed it sure looked that way.

“You can start to make the case for it,” said Nash, before adding, “Ky’s as likely to give anyone else in the league 40 as he is the Celtics.”

It was his fifth 40-point game with the Nets, already third-most in team history. After he had punished his former teammates — following a highly scrutinized free-agent exit in June 2019 — Irving shared some moments on-court with them.

“Big surprise to a lot of people. All that [crap] talking about me and all the relationships I have with every teammate of mine. I’m glad we get to see it,” Irving said. “I’ll let the casual fans talk about what it could mean in terms of a special night, I’m waiting for the main stage, playing in front of millions and it actually mattering in terms of win or lose or go home. I’m looking forward to that.”

James Harden flirted with yet another triple-double (22 points, 10 rebounds and eight assists), while Shamet added 18 on 6 of 9 from deep.

Even with Kevin Durant still out, the Nets improved to 17-3 with at least two of their Big Three playing. They overcame Jayson Tatum’s 31 points, as well as second- and third-quarter deficits behind Irving.

“He’s just different. He’s a different breed,” Harden said. “He has that killer mentality in a sense of, no matter who we play or where we’re playing, he is going to go out there and try to destroy the opponent and that’s something that you’d want on your team at all times.

“That mentality is what sets him apart from a lot of guys in this league.”

The Nets stumbled out of the gate, missing eight of their first 11 shots — all three from behind the arc — and falling behind 18-8 after nine quick points by Kemba Walker in the first 6:19. But he didn’t score again until late in the fourth.

The Celtics lead got to 27-16 after a Payton Pritchard 3-pointer with a minute-and-a-half left in the first quarter.

But with the Nets down 38-33 on Tristan Thompson’s dunk with 7:57 left in the half, Brooklyn strung together an 18-7 run to take the momentum, flipping a five-point deficit into a six-point cushion.

Irving had 10 points and a steal in the run, and his pull-up 3 put Brooklyn ahead 51-45 with 3:46 left in the half.

Trailing again 70-65 in the third quarter, the Nets reeled off a 23-12 run that put them ahead for good.

Irving had seven points in the run, while Harden added five with a pair of assists for an 88-82 lead.

When the Celtics climbed within 108-106 on a Tatum reverse, Irving sparked the 13-3 run to end it.