Throughout 72 games, the Celtics have seldom strayed from their winning mentality. Despite having already clinched the top seed in the Eastern Conference and the greatest record in the NBA, they continue to crush their opponents, winning by an average of 11.5 points per game.

On Monday night in Atlanta, everything appeared to be business as usual.

Despite Derrick White and Jrue Holiday’s absences, the Celtics dominated a Hawks club without seven key players, including Trae Young, Jalen Johnson, Onyeka Okungwu, and Saddiq Bey.

A little more than three minutes into the game, the Celtics had already created a double-digit advantage, and by 4:41 of the first period, Boston had doubled up the Hawks on the scoreboard, leading 30-14.

Jayson Tatum scored 23 points in the first half, Kristaps Porzingis added 15, and Jaylen Brown added 13. In the second quarter, the gap grew to 30 points. This scenario was all too familiar for the Celtics, who had found themselves in similar dominant positions several times this season, with 16 30-point leads and 14 10-point deficits.

Complacency has not been an issue in these circumstances. The Celtics continue to play their game and do not let up, as is their mindset. However, Monday night was an unusual exception. Indeed, the phrase “took their foot off the gas” was used multiple times after the game.

“We were up 30 and kinda took our foot off the gas, and it cost us,” Brown said. “We got a little casual, we got a little distracted, and it cost us.”

“Tonight was, obviously, tough,” Tatum added. “We kinda let our foot off the gas in the second half, and they got into a great rhythm.”

While such lapses are rare for the Celtics, Monday night served as a sobering reminder of the consequences of complacency. They let their guard down, allowing laziness and distraction to set in, and ultimately paid the price by squandering a 30-point lead and succumbing to the Hawks 120-118.

Prior to Monday night, Boston had been 130-0 in the regular season since 1996 when leading by 30 points or more.

“I think it was just the margins,” head coach Joe Mazzulla said postgame. “It’s a good lesson—when you let a team hang around. They made those margin plays: points off turnovers, three-pointers, offensive rebounds. Credit to them for battling back.”

The turnaround began near the end of the first half. The Hawks finished the first half with a 16-6 surge, trimming Boston’s lead to 18. The run appeared innocuous at the moment, but the Hawks came out blazing after the half, extending it to 30-7 and trimming the advantage to three at 4:48 in the third quarter.

Although the Celtics increased their advantage to 11 after a strong third quarter from Payton Pritchard, Atlanta’s 30-7 run provided all the momentum they needed. Atlanta proceeded to come back, outscoring Boston 64-44 in the second half. Boston scored 44 points in the second half, matching their total from the first quarter.

“I thought we got some good looks that we missed,” Mazzulla said on what went wrong with the offense in the second half. “But I thought their physicality. They were extremely physical as soon as the ball got over half-court. I thought that messed up our offense. They made us work for everything. I thought they defended us pretty well, and I thought we missed some really good looks.”

Basketball is a delicate game. You have to play the game the right way and with the right intentions for an entire 48 minutes. The Celtics experienced the weight of this truth firsthand in last year’s postseason, and Monday night’s setback served as a stark reminder.

“Not overreacting,” Porzingis said on the biggest thing the Celtics can take from the loss. “It’s the NBA. These kind of things happen. We just don’t want to make this a habit. And it hasn’t been a habit for us. We slipped one game. We relaxed a little bit, and we paid the price.”

“You win, or you learn,” Porzingis added.

His assessment is accurate. This hasn’t been a common theme for the Celtics. Mental slippage is inevitable across an 82-game slate. And while you never want to be on the wrong end of a blown 30-point lead, this is a humbling reminder that winning a championship requires attention to detail night in and night out for an entire 48 minutes.

“We expect to win every game,” Porzingis said. “But I like that we have some bumps heading into the postseason.”

Overall, the Celtics have done an excellent job responding to and learning from their missteps. They are an NBA-best 12-2 after a loss this season. When the Celtics watch the tape in their Atlanta hotel, they should be able to glean some useful information from it.

“Stuff like this is good to watch, see, learn, inspire, and motivate,” Brown said after the game. “We need to learn from our mistakes, and, hopefully, that puts a battery in our back coming out next game.”

Here are four takeaways as the Celtics’ nine-game winning streak ends in Atlanta.

Beat them at their own game.

The Celtics have been unstoppable from beyond the arc this season, with an incredible +840 scoring edge heading into Monday night’s game. Their effectiveness from three-point range has frequently been a determining factor, as evidenced by their incredible 44-4 record when they shoot more threes than their opponents.

It’s no secret that three-pointers are important in today’s game, and the Hawks dominated that area on Monday night. De’Andre Hunter, Viti Krejci, and Bogdan Bogdanovic each hit four three-pointers, helping the club shoot 18-of-36 (50%) from downtown.

Atlanta made 11 of 19 shots in the second half (57.9%).

In contrast, the Celtics struggled to establish their rhythm, shooting only 1-of-15 (0.6%) from beyond the arc in the second half. Their lone three-pointer set a season low for made triples in a half, leaving them with a massive 33-3 perimeter deficit.

“That’s the weapon of the three-pointer,” Mazzulla said after the game. “They shot 50 percent from three. It alters everything in the game. And we shot 28 percent.”

The Celtics concluded the night 11-of-38 (28.9%) from deep, slipping to 3-8 when they shot less than 30%. Such occurrences are unusual enough to not constitute a serious worry, especially when postseason series are seven games long.

Slowing down.

The Celtics do best when they get out and run in transition. They strayed away from that in the second half, as evidenced by their 44-point total.

This was made clear when Jayson Tatum committed an eight-second violation at the 8:26 mark of the fourth quarter, after the Hawks had taken their first lead.

Slowing down the game does not help the Celtics’ success, as evidenced by their second-half performance on Monday night. Their attack was disjointed, relying on isolated plays that produced only four assists—the second-lowest total in the previous ten years.

Boston committed more turnovers (5) than assists in the second half.

Mazzulla routinely emphasizes the significance of connectedness in the game, and Monday night’s game demonstrated how Boston’s defensive mistakes hampered its offensive flow.

Late-game execution

In the last 2:40 of the fourth quarter, the Celtics were outscored 11-6. With the setback, the Celtics fell to 12-11 in one-possession games under two minutes, and 4-7 in games decided by one point in the last minute.

Criticism of the Celtics’ late-game execution is not new. They have often struggled in crunch-time scenarios, and they need to better at these critical moments.

Slowing down is a contributing factor to the problem.

After Dejounte Murray made a reverse layup with exactly one minute remaining to give Atlanta a one-point lead, Jaylen Brown carefully dribbled the ball up the floor. The Celtics ran out of time while nothing happened, prompting Jaylen Brown to attempt a contested 25-foot stepback three that clanked off the front rim.

Murray collected the rebound, controlled the time, and pulled up for an 18-foot jumper. The shot failed, but Clint Capela got the rebound and kicked it out to De’Andre Hunter, who could have dragged out the clock and forced the Celtics to foul, but instead he jacked up a three from the top of the key, sticking the dagger in the Celtics.

While the offensive rebound is unsatisfactory in this case, it returns to the previous possession.

The Celtics have become accustomed to standing about and watching guys iso as the shot clock ticks away down the stretch.

In fairness, every team plays slower in the fourth quarter, and the fact is that the best shot you’ll get is a contested three-pointer or a fadeaway. However, with the tools they have, the Celtics have some leeway to be a little more creative. Furthermore, data suggest that Mazzulla and the Celtics are among the league’s top scorers following a timeout.

Leveraging this expertise, especially in late-game scenarios, could benefit the Celtics in the long run.

Jaden Springer makes the most of his minutes.

Jaden Springer, who joined the Celtics from the 76ers at the trade deadline, hasn’t played many major minutes off the bench. However, the 21-year-old had a significant impact on Monday night’s game, playing more than eight minutes in the fourth quarter and making valuable contributions.

While his box score stats aren’t particularly impressive (two points, a rebound, and a block), the Tennessee native provided the Celtics with a defensive spark and a surge of enthusiasm.

“He brings great physicality, his ball pressure,” Mazzulla said after the game about what he saw from Springer. “It’s just an opportunity for him to grow, his physicality and his defensive presence.”

The 6-foot-4 guard demonstrated physicality and persistent ball pressure. Brad Stevens’ decision to trade for the former first-round pick at the deadline was understandable, as he demonstrated the ability to be a great defensive player.