Nets guard Kyrie Irving is pressured by Celtics forward Grant...

Nets guard Kyrie Irving is pressured by Celtics forward Grant Williams and guard Marcus Smart in the first half of an NBA game at Barclays Center on Thursday. Credit: Kathleen Malone-Van Dyke

The Nets need to find some offense.

That was the biggest take-away Thursday night as they began what is projected to be a monthlong stretch without Kevin Durant, their leading scorer.

The Nets scored only 16 points in the fourth quarter as the Celtics broke open a tight game and came away with a 109-88 victory at Barclays Center.

“Give them credit. Their defense took a step up and we couldn’t produce on the offensive end of the floor,” Vaughn said. “They were challenging every shot of ours and making every shot tough for us.”

The Nets were hindered by a strangely lopsided performance by Ben Simmons. Though Simmons had 13 assists, including 10 in the first half, and grabbed nine rebounds, he failed to score a point and took just three shots.

At one point early in the fourth, Simmons was booed by fans for passing the ball instead of taking what looked like a sure shot. It marked the second time this season that Simmons went scoreless as he also did so on Nov. 28 against Orlando.

Simmons said after the game that he knows he has to try to produce more offensively.

“I think I’m giving the ball up a few many times,” he said. “I know who I am. I know I need to get to the rim. I know I need to get buckets. That’s going to help my teammates, help them get going.”

The Nets (27-14) have lost two of their last four after winning 12 in a row. The Celtics (31-12) have the best record in the NBA.

Kyrie Irving led the Nets with 24 points and Joe Harris added 18. Boston, led by Jayson Tatum’s 20 points, had six players in double figures.

A week ago, this matchup looked as if it would be the ultimate showcase of two Eastern Conference heavyweights.

Boston was the last team to badly beat the Nets, who entered last night having won 14 of their last 15 games since losing to the Celtics, 103-92, on Dec. 4.

Then Durant, who had been playing all season at an MVP level, suffered an isolated MCL sprain of his left knee Sunday in Miami. He is expected to be out a month, meaning he will miss at least 14 games.

The matchup got a little more even-sided shortly before tipoff when Celtics coach Joe Mazzulla announced that Jaylen Brown, who is averaging a career-high 27.2 points, had been downgraded to out because of an abductor strain. Fellow starter Al Horford was also out because of low back stiffness.

For three quarters, neither team was able to take a double-digit lead. Then the Celtics opened the final quarter with a 12-2 run, from which the Nets couldn’t recover.

At that point, the Nets were so desperate for offense that Vaughn had to take Simmons off the floor and replace him with Seth Curry in crunch time.

Irving, the Nets’ sole remaining superstar, took more than half of the team’s shots in the final quarter, scoring six points, but shooting only 3-for-10.

“I think overall we’re going to go to Ky, especially in the fourth because we know he can get a shot,” Vaughn said. “Marcus Smart’s a pretty good defender and they’ve got guys at the rim when Ky got to the rim. I think he had some looks that I’ve seen him make before.

“So I totally trust his ability to make shots for us but make the right decision at the end of the game, also.”

Still, Irving is going to need some help if the Nets are going to get through the next 13 games or so without Durant.

Said Irving when asked where Durant’s points are going to come from: “Each and every one of us. Every single day. It’s just gonna be a collective effort.”

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