The Celtics failed to emphasize lead protection when it mattered the most against a Trae Young-less Hawks team on Monday night, turning a potential 10-game Boston winning streak into a much-needed learning lesson.

With an already-secured No. 1 seed in the Eastern Conference, the Celtics don’t have much left to play for besides maintaining the right momentum. And for nearly half of regulation that’s exactly what Boston conveyed, toying with an undermanned Atlanta team to take a 30-point lead midway through the second quarter. Typically, that signals a well-earned early clock-out for Jayson Tatum and Jaylen Brown, but that was far from the case at State Farm Arena.

The Celtics allowed the Hawks to fight back and escape with a 120-118 win, turning the closing seconds of the fourth quarter into a nail-biting race to the finish line. On the same night in which Boston opened the first quarter with a 44-point outpour to keep Atlanta at bay, the Celtics followed by scoring 44 points in the third and fourth quarters, marking their weakest second-half response recorded this season.

“We got off to a really good start, but it’s a good lesson,” Celtics head coach Joe Mazulla told reporters, per NBC Sports Boston. “You let a team hang around and they made those margin plays. Points off turnovers, 3-pointers, offensive rebounds and credit to them for battling back.”

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Mazzulla added: “We played them last year in the (playoffs), it was those margins. It was defend the 3-point line. It was limiting them to one shot and I think we still outshot them so it’s weird. We just gotta learn from that.”

Boston was without Jrue Holiday and applied minimal defensive urgency, giving the Hawks the green light to push the Celtics to their limit. Atlanta’s DeAndre Hunter put the nail in Boston’s coffin, draining a game-sealing three to put the Hawks up four points with 9.2 seconds left in regulation. However, that resulted from three Celtics players failing to secure a defensive rebound against Clint Capela in the game’s biggest moment.

That play perfectly summarized Boston’s negligence in maintaining a big double-digit lead by playing on both sides from start to finish. It provided the Celtics with a deja vu experience to last season’s playoff series with the Hawks in which Boston allowed a significantly less talented Atlanta team to push a first-round matchup to six games.

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There’s no leeway for excuses this go-around for performances alike to creep their way into this year’s playoff hunt, and the team knows that.

Boston went 0-for-0 from three in the third quarter then 1-of-6 in the fourth, finishing an inefficient 11-of-38 (28.9%) by the final buzzer.

“We just took our foot off the gas and they just got confidence, and at that point, it’s tough to beat anybody when they got it rolling,” Brown said postgame, per NBC Sports Boston. “… I think we have the right approach, we’ve had the right approach. Tonight wasn’t the best example of that. We kind of took our hands off the steering wheel a little bit. And no matter if we up 30 (points) or we down five, we gotta have the same approach and the same mindset.”

Even after the shocking collapse, Boston owns an NBA-best 57-15 record, but can’t succumb to the bad habits that prevented them from returning to the Finals last season.

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Featured image via Brett Davis/USA TODAY Sports Images