4 takeaways as the Celtics lose to the Hawks in a stunning NBA Cup upset

The Celtics blew a 15-point second-half lead and fell 117-116 to the inferior Hawks on Tuesday night, winning their first group game in Group C of the Emirates NBA Cup. The Celtics entered the game as 16.5-point favorites.

Boston shook off a sluggish start and seemingly took control of the game in the second quarter, building a double-digit lead at halftime behind 20 points from Jaylen Brown and 14 points from Derrick White. The Hawks played without four rotation players for the match, including All-Star guard Trae Young, which led to them starting two-way guard Keaton Wallace and playing at a disadvantage all night.

The Celtics led by as many as 15 points in the second half, but the Hawks erased that deficit, dominating the field with a 68-32 advantage from deep over Boston and 18 second-chance points. Onyeka Okongwu scored a go-ahead goal for Outback in the final minute, while Jaylen Brown (team-high 37 points) missed a potential game-winner as time expired. Derrick White also added 31 points in the hosts’ setback, while Dyson Daniels’ 28 points led five different Hawks in double figures.

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Boston had its two-game winning streak snapped and was 9-3 on the year. Boston’s next NBA Cup game is next Tuesday night at TD Garden against the Cleveland Cavaliers.

The Celtics will make a quick turnaround on Wednesday night, taking on the Brooklyn Nets at Barclays Center for the second time this month.

Here are four takeaways about the Celtics on Tuesday night:

Jaylen Brown finds his offensive groove after hip problems: The All-Star struggled with his offensive rhythm in his return from a four-game absence in Milwaukee, going just 4 of 13 from the field in 37 minutes. Brown looked far more comfortable against a lackluster Hawks defense. He scored 16 points in the first quarter and led all scorers at halftime with 20 points while shooting 7 of 12 from the field. The small forward did most of his damage from the field early, but knocked down two key three-pointers in the fourth quarter. Brown also celebrated his joy in the game against Atlanta by reaching the free throw line ten times and scoring 11 of 15 goals from the interior line. He struggled with his free throws late in the fourth quarter (6 of 10).

A surprising rotation tweak from Joe Mazzulla: Traditionally, the Celtics have split the rest periods of their two stars. For most of this year, Tatum has played the entire first quarter while Brown takes a break in the final minutes of the frame. However, because Brown showed such great rhythm in the first quarter on Tuesday night, Mazzulla opted to play the entire first quarter with his two stars, which was the first time in their regular-season careers, according to NBC stats guru Dick Lipe. The substitution worked well as Boston’s reserves held their own alongside Derrick White and Jrue Holiday early in the second quarter. It will be interesting to see if this was an anomaly or an option that Mazzulla could use more regularly to help his stars find an early game rhythm.

The Celtics are dealing with a rare bout of sloppiness that is bothering them: The Celtics saw their 15-point second-half lead blown midway through the fourth quarter and had only themselves to blame for their woes. The team recorded a high of 20 turnovers this season, a very unusual number considering the previous high was 15 this year. Most of these errors were due to careless passes, and Boston’s stars were the main culprits for these problems, piling up nine turnovers combined. The Hawks scored 19 points off those Boston turnovers, allowing them to stay in the game all night despite a difficult 3-point shooting night.

The Hawks may have already won the Dejounte Murray trade: It’s only been ten games and an injury to Murray with the Pelicans might impact my judgment here a bit, but it’s hard not to like what Atlanta got in the deal for Murray. Dyson Daniels already looks like a goalie at age 21 due to his defensive abilities despite a reliable 3-point stroke. With two more first-round picks and a serviceable replacement in Larry Nance Jr., it’s a great return to break down a backcourt duo in Murray and Trae Young that wasn’t going anywhere anytime soon.