Evan Mobley says the Boston Celtics were a "good test" for the Cleveland Cavaliers as they saw their perfect start to the NBA season ended.

The Cavaliers were 15-0 going into the game, the fourth team in NBA history to open with that record, but a slow start cost them as they lost 120-117 to the defending champions on Tuesday.

Cleveland fought back to cut a 21-point deficit to two in the third quarter, with Donovan Mitchell making it a four-point game with 24 seconds remaining, but they could not snatch a win as Al Horford's late dunk sealed it.

Mobley had 22 points, 11 rebounds and six assists for the Cavaliers, and chose to focus on the positives from the defeat, in particular, their response to the half-time deficit.

"I'm happy how we fought back," said Mobley. "That was the main thing that I can be happy about.

"I feel like we didn't start the best. That was the main thing that really shot us in our foot. But overall, I feel like we played a pretty good game.

"I think it was a good test, and from what I saw out there, we could beat anybody."

Mitchell, Cleveland's top scorer with 35 points, added: "[I learned] that it's the group I thought we were.

"It's just a test. This is a big test. Whether you win, lose or draw, it's just a test, and obviously, you want to learn through wins, but we lost tonight.

"But there's a lot we can take away from the good in the second half and a lot we can learn from the first half to get better at."

The Celtics bounced back from a defeat to the Atlanta Hawks in the first round of the NBA Cup with this result and made it three wins on the trot.

They improved their record to 12-3, though they still sit below the Cavaliers in the Eastern Conference, and Derrick White says Boston knew a lot was riding on this result.

"Everybody knew that they were undefeated and coming to our building," he said. "And also, it's an NBA Cup game, and we already lost one game.

"So, it was a really big game in that aspect. So, we were all aware [of the excitement around it]."