Cleveland Cavaliers guard Donovan Mitchell drops to the floor after colliding with Boston Celtics forward Jayson Tatum (0) during the second half of an Emirates NBA Cup basketball game, Tuesday, Nov. 19, 2024, in Boston. (AP Photo/Charles Krupa)

BOSTON — This one wasn’t about their perfect season or a 15-game winning streak or even the NBA Cup.

This was a chance for the undefeated Cleveland Cavaliers to see where they stood against the defending champions.

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“This was a great test for us, and unfortunately we didn’t get the win,” said Donovan Mitchell, whose 35 points couldn’t prevent the Cavaliers from picking up their first loss of the season, 120-117 to the Boston Celtics on Tuesday night.

READ: NBA: Celtics end Cavaliers’ 15-0 start

“It’s definitely a measuring stick,” said Mitchell, who scored 18 points in the fourth quarter, including 14 straight Cleveland points in the final four minutes. You want to see where you’re at, but not hold too much weight on it. … We’re not going to be the same team now that we are in April.”

The Celtics won an unprecedented 18th championship last spring — rolling over the Cavaliers in five games in the Eastern Conference semis – and entered the season as the favorites to repeat, or at least make it back to the NBA Finals. Cleveland, which won a playoff series last year for the first time since LeBron James left the second time in 2018, wasn’t expected to be a contender.

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But it was the Cavaliers who shot to the top of the standings this fall, with a franchise-record 15-game winning streak that left them as the last unbeaten team in the league. Coach Kenny Atkinson was the first NBA coach ever to win his first 15 games with a new team.

“You want to be a part of history in that regard. But it’s a thing of the past. It’s over with. And, you know, it was a good run,” Mitchell said. “It was fun. When you’re playing basketball, good basketball, and obviously winning games — whether it’s blowouts, close games, come-from-behind wins — you enjoy these moments.

“It’s great to be part of history. We wish we had kept it going,” he said. “But, like I said, there’s no championships in November.”

The Celtics used a 3-point barrage on Tuesday night to open a 21-point lead, then watched as the Cavs cut it to two in the third quarter. Cleveland trailed by nine, 114-105, with 90 seconds left before Mitchell hit a floater and then another layup to cut the deficit to five points.

With Boston up 117-110 and 25 seconds left, Mitchell hit a rainbow 3-pointer to make it a four-point game. After Jayson Tatum, who scored 33, hit one of two free throws, the Celtics star bowled Mitchell over while going for a steal.

READ: NBA: Cavaliers’ perfect start on line vs champion Celtics

Mitchell stayed down on the court for a few minutes, but the play was not reviewed to determine if it was a potential flagrant foul that would give the Cavaliers a pair of free throws and the ball.

“I was just praying to God I didn’t have a concussion. That’s my only thing,” said Mitchell, who watched the play again after the game and didn’t think it should have been ruled a flagrant foul. “It was a play on the ball. He’s got bony shoulders. So I hit his shoulder, and it is what it is.”

Atkinson blamed himself for not having an answer for a Celtics team that went 14 for 22 from 3-point range in the first half to open a 17-point lead. And he praised his team, which faced the Celtics without Issac Okoro (ankle), Caris LeVert (knee) and DeanWade (ankle), for erasing almost all of that deficit in the third quarter.

“They shot the heck out of it,” Atkinson said. “That big second quarter, that was too much to overcome. Second half, we turned it up, but it’s kind of too late.”

And the coach was already looking to their next matchup, in Cleveland on Dec. 1.

“They’re so good. We gave resistance in the second half, but … a lot of stuff we can improve on,” he said. “We’ll have another shot at them soon.”