BOSTON — The Nets are officially a losing team. 

The Celtics knocked them into the land of the losing, handing Brooklyn a 126-120 defeat before a sellout crowd of 19,156 at TD Garden — many of whom booed Kyrie Irving’s every touch and chanted “Kyrie sucks!” in the waning moments. 

Those waning minutes were a bitter pill for the Nets to swallow. Kevin Durant (37 points) had played brilliantly to give them a 111-110 lead with 3:15 left, but they conceded a 13-4 run to stumble to ninth in the East (losing a tiebreaker with the Hornets), and even deeper in the play-in. 

And it wasn’t hard to pinpoint what cost them on Sunday. 

“Jayson Tatum,” Irving said of his ex-teammate. “They just start picking the scab, start running the same play over and over again, getting switches and they were just punishing us down the stretch.” 

Durant might be the best player on the planet, but Tatum was the best player on the court Sunday. He poured in a game-high 54 points, getting stronger and stronger with 16 in the third quarter and 18 in an utterly dominant fourth. 

“He made some tough 3-point shots over top of us,” Durant said. “He’s an elite shot-maker.” 

The Nets allowed 38 free-throw attempts (to their 20), saw Boston shoot 17-for-36 from deep and got a comparatively quiet outing from Irving. After eviscerating his former team last season, he had just 19 points. 

“I know it’s going to be like that for the rest of my career coming in here,” said Irving, who spent two seasons in Boston. “It’s like the scorned girlfriend who just wants an explanation on why I left but still hoping for a text back, and I’m just like yeah — it was fun while it lasted.” 

With Irving and Durant together for just the fourth time this season — and first as a duo alongside Seth Curry and Andre Drummond — the Nets showed they didn’t have Boston’s level of cohesion. With just 17 games left, how long will that take? 

“Man, I wish I had an answer for that,” Irving said. “But we’ve all accepted that it’s been a funky year. It’s been a lot going on, in and out of the lineup. We have one of the most changing lineups in the league, us and the Lakers. So it just shows when you don’t have that consistency and you don’t have guys in the lineup it takes a big hindrance on everything; and I take my accountability for that as well. 

“So now we just have 17 games left, and we make the best of it. And we’re not in this do-or-die mode, but we know in order for us to be in position for contention we’ve got to do the little things in order to win every single night, and I’m gonna have to show up even more so.” 

The Nets (32-33) need Irving to show up Tuesday. They have the same record as the eighth-place Hornets and are just percentage points ahead of the 10th-place Hawks, and a huge visit to Charlotte looms on Tuesday. 

Brooklyn was three full games behind Toronto and five behind Cleveland for the sixth and final guaranteed playoff spot, with the Raptors and Cavaliers facing each other Sunday night. It’s could be approaching time to worry. 

“Concerned and worried, those words, it’s easy to do that. It’s easy to be concerned, it’s easy to be worried and pissed off and mad. It’s easy to do that,” Durant said. “We can point fingers and blame our season on other things. But let’s just go play.” 

In a barnburner that had a playoff vibe, 28 lead changes and 19 ties, the Nets were up 111-110 after Durant found Curry for a 3. But they conceded a 13-4 blitz. 

Tatum’s free throws, followed by Jaylen Brown (21 points) swiping a Durant pass and Tatum’s ensuing layup gave the Celtics a 114-111 lead. 

Brown beat the shot clock with a dagger 3 with 39.4 seconds left. And he capped the run at the stripe eight seconds later, putting Brooklyn in a 123-115 hole.