BOSTON — Kyrie Irving returned to Boston on Friday concerned about racism and belligerence from the crowd. It looks like the Nets should have been more worried about the belligerence the Celtics had in store for them.

There was no spittle sprayed at Irving, and no shower of racism hurled at his Nets. Just raucous boos from the stands and aggressive physicality from the Celtics, who handed the Nets a 125-119 loss in Game 3 of this first-round playoff series.

Even with capacity limited to just 4,789, the crowd made Irving’s first game in front of Celtics fans since his 2019 Boston departure a bitter one. It will only get tougher in Game 4 on Sunday.

“It was a great start to seeing what this environment’s going to be like and just looking forward to the challenge,” Irving said.

“It’s basketball: I’ve been in a few environments in my life. So, like I said, as long as it’s just strictly the nature of basketball out there, it’s nothing extra, I’m cool with it. Going to get some preparation with the team [Saturday] and then get prepared for Sunday. But want to keep it straight basketball.”

The racial lines Irving spoke about earlier this week never got crossed. The Nets’ own poor defense and lack of physicality were more of an issue than the behavior of the crowd.

“Not one bit of a distraction. That wasn’t something that even crossed our minds,” James Harden said. “We have to play better as a unit, and we will Game 4. This thing isn’t gonna get easier for us. It’s gonna be difficult.”

It will be if the Nets’ Big 3 play like a Big 2.5 again.

Harden was stellar with 41 points, 10 assists and seven rebounds. Kevin Durant added 39 points and nine boards.

But Irving was far quieter than the crowd that booed his every touch, finishing with 16 points — just two in the first half — as he was greeted with chants of, “Kyrie sucks,” “[Bleep] you Kyrie” and all sorts of other taunts.

Jayson Tatum, held to 28.1 percent shooting in the Nets’ wins at Barclays Center, erupted for 50 points. Marcus Smart added 23 while Tristan Thompson (19 points, 13 boards) took it to Brooklyn physically at every opportunity.

The Nets coughed up a 17-2 spurt spanning the first and second quarters — part of an extended 36-15 Celtics run — and never led again.

“They took the momentum, their crowd with them,” coach Steve Nash said. “They started to feel a little confidence for the first time in the three games and we didn’t get enough stops.”

After trailing by as many as 10 points, the Nets did pull even at 75-all with 5:23 left in the third quarter on Durant free throws. But seconds later, Irving fouled Smart on a four-point play to open the floodgates.

The Celtics blew the game open with a 21-4 run. By the time Tatum capped it with a fadeaway, Boston led 96-79 and it was essentially over.

“We stopped guarding. When we stop guarding, teams are going to shoot the ball and get hot,” Harden said. “We have to keep our foot pressed down, especially defensively. Early in the game we had the energy, we had the juice; and we gave them confidence.”

It could prove costly. The Nets lead the series 2-1, but Game 4 will be played with the arena at near full capacity, and Tatum has finally broken out.

After the Nets hit six of their first seven shots to take a 19-4 lead on a Joe Harris 3-pointer, Boston didn’t blink.

The Nets still led 32-23 before a 12-0 Celtics run spanning the first and second quarters dropped them behind by three.

Irving had just two points on 1-for-5 shooting at the break. His jumper put the Nets up 73-72 midway through the third.

After Durant pulled them even at 75-all, the Nets imploded. They shot just 1-for-8 in the run, while Boston went 6-for-7, picking the Nets apart with the 1-4-1 pick-and-roll.

“It was big momentum plays, Tristan hitting the offensive glass and keeping plays alive,” Irving said. “Just the little details of the game where we felt like we could’ve been there in terms of putting a body on guys. We’ve got to be ready for that for Game 4.”