Celtics allow a 28-point first-half lead to slip away, fail to bounce back, and lose to the Nets

Jaylen Brown put up a shot over the Nets' Dorian Finney-Smith in the first quarter, which ended with Celtics holding a 22-point lead at 37-15.Barry Chin/Globe Staff

One month ago, this Celtics’ matchup against the Nets figured to be filled with juice and buzz. But then Brooklyn traded away superstars Kevin Durant and Kyrie Irving, and its team was essentially left without an identity.

The Nets entered Friday night’s game at TD Garden riding a four-game losing streak, facing a Celtics team that remains the favorite to win the NBA title. And for the first 15 minutes, the teams played their parts. The Celtics roared to a 28-point lead and appeared to be toying with the seemingly overwhelmed Nets, who offered limited resistance against drive to the rim.

But there were some signs that Boston was taking the opponent lightly. Jaylen Brown tried to throw an alley-oop to Robert Williams off the backboard. Crisp ball movement turned into players hunting for their own shots, because it appeared to be a good night to pile up stats.

Gradually, the Celtics’ carelessness began to swell, as did the Nets’ confidence. By night’s end, long after the massive lead had been flipped into a substantial deficit, boos poured down from a crowd that no longer had anything to cheer about, and the Celtics were handed perhaps their humbling loss of this season, 115-105.

“Just everything happened so easy there at the beginning of the game,” Derrick White said, “and then they just started playing harder.”

With the loss, the Celtics slipped a full game behind the idle Bucks for the top spot in the Eastern Conference.

Afterward, Celtics coach Joe Mazzulla’s frustration level appeared quite low, a stark contrast from last season, when former coach Ime Udoka routinely and publicly blasted his team on nights when everything unraveled. Instead, Mazzulla mostly focused on statistics, pointing out that the Nets had more 3-pointers, free throws, offensive rebounds, and overall shots — and fewer turnovers.

“I’ll be more upset if it happens again,” Mazzulla said, “if we’re in another situation.”

Regardless, the night continued a trend of uneven performances since the All-Star break, with some close wins potentially masking other flaws. The Celtics escaped with an overtime win against the sub.-500 Pacers, beat the 76ers on Jayson Tatum’s last-second 3-pointer, were crushed by the Knicks, and then let much of a 21-point lead slip away against the Cavaliers before winning by 4. Now, there is this collapse.

“We let them get going, let them get some confidence,” Brown said. “and at that moment, when the momentum shifted, they became hard to stop.”

Brown had 35 points. Tatum, who turned 25 on Friday, added 22 points and 13 rebounds, but he missed all eight of his 3-point attempts and had four of Boston’s 19 turnovers. Williams left the game in the third quarter due to hamstring tightness and did not return. Malcolm Brogdon sat out due to ankle soreness.

Mikal Bridges erupted for 38 points to lead the Nets, who didn’t even have a scorching shooting night to steal this improbable win, as they shot just 31.8 percent from the 3-point line.

But Mazzulla stressed several times that the fact that the Nets simply attempted more 3-pointers had concerned him throughout the night. The Celtics were 9 for 30 from the arc, but made 59.3 percent of their two-point attempts.

“[We’ve] just got to, especially with the lead, just double down on things that got us the lead,” White said.

Boston shot 56.5 percent from the field in the opening quarter and roared to a 37-15 lead. The night seemed set up for the stars to get some well-earned rest during a blowout win. The start of the second quarter was more of the same, with the Celtics taking a 51-23 lead, their largest, on a Brown layup with 7:23 left.

But Boston’s defense softened and lacked the urgency needed to finish off another NBA team. Brooklyn erupted for 32 points over the final 7:11. After a Celtics backcourt violation with 2.9 seconds left, Spencer Dinwiddie found Dorian Finney-Smith for an alley-oop layup at the buzzer, capping a 40-point quarter and improbably pulling the Nets within 64-55 at the break.

“If you don’t end the quarter well, you give a team life,” Mazzulla said. “You don’t end a half well, you give a team life.”

For the Celtics, things deteriorated even more severely during the third quarter. Bridges ignited a 27-10 run with a 3-pointer and ended it with a putback and a pair of free throws. When the Celtics went to their bench for a timeout near the end of that burst, the crowd that had been loving the show an hour or so earlier began to boo.

Boston trailed by 5 points at the start of the fourth, but a Tatum turnover led to a Cam Johnson layup as he was fouled, and that was followed by a Bridges 8-footer that put Brooklyn in front, 100-88, with 8:09 left. The Celtics briefly flirted with comeback attempts, but none materialized.

“We got off to a really good start playing the right way,” Mazzulla said. “And we just let the foot off the gas and it cost us.”