CLEVELAND — When the NBA schedule was released last summer, this November road game against the Cavaliers figured to be a good early gauge of where the Celtics stood in their quest to ascend to Cleveland’s level.

But the team that arrived here in the wee hours Thursday morning does not resemble the one that the Celtics had constructed. Al Horford (concussion), Jae Crowder (sprained ankle), and Kelly Olynyk (shoulder rehabilitation) remained back in Boston, giving coach Brad Stevens a patchwork group to try to take down mighty the world champions.

For the most part, patchwork groups do not work against LeBron James and the Cavaliers. And for long stretches at the start, that reality seemed quite obvious.

But after trailing by 20 points in the third quarter, the Celtics crafted a spirited and unlikely comeback, as a 3-pointer by Jaylen Brown with 6:17 left improbably pulled Boston within 5 points. In the end, though, that is where the surge stalled, as the Cavaliers held on for a 128-122 win.

“You’re supposed to compete for the full 48 [minutes],’’ Stevens said. “We weren’t as good in the first half, so we dug quite a hole. So I’m glad we [came back], but that doesn’t excuse playing poorly at times. I didn’t think we took very good shots at different times and I thought our transition defense in the first half was porous, and there’s no excuse for that, no matter who’s available.’’

Isaiah Thomas had 30 points and six assists to lead the Celtics. Avery Bradley added 26 points and 10 rebounds, and Brown had 19 points and five rebounds.

James finished with 30 points, 12 assists, and 7 rebounds to lead the Cavaliers.

For the Celtics, the night’s most encouraging development was the play of Brown, the rookie forward who received his first pro start in place of Crowder and was tasked with both guarding James and trying to score on him. He was effective in both areas in the first half.

On offense, he showed patience as he looked to attack the rim. With 1:22 left in the second quarter he drove the right baseline on James and threw down a two-handed dunk.

“Twenty-year-old going against arguably the best player in the world,’’ Thomas said. “He didn’t show no back-down.’’

On defense, Brown tried to bother James with his quickness. James made just 2 of 6 field goals before halftime, although he shredded the Celtics for 20 points in the third quarter. Brown said he wanted to keep James out of the interior, where he is so dangerous as a scorer and a distributor.

“I mean, he’s the best player in the world,’’ Brown said. “He’s going to hit some tough ones, and he did hit some tough ones.’’

Perhaps Brown’s greatest weakness as a freshman at California was his outside shooting. In this game he hit 3 of 6 3-pointers. He said he has been working diligently with assistant coach Micah Shrewsberry to develop that part of his game.

Of course, the Celtics did not just come here to witness Brown’s maturation; they came here, shorthanded or not, to win.

“Very disappointing,’’ Thomas said. “There’s no excuses. We played a game we felt we could have won, and we only lost by 6. But we have to do a lot better in certain situations.’’

The Cavaliers surged to a comfortable lead by making 11 of 18 3-pointers in the opening half.

“I actually thought in the halfcourt they made some where we were really good and really contested and were right there,’’ Stevens said. “The ones that kill you are the ones in transition and you get consumed with what’s going on with the ball.’’

With Crowder and Horford out, there was even more onus on Thomas to score, but he did not convert his first field goal until 4:30 remained in the second quarter. Cleveland took a 68-51 advantage to halftime.

With 9:25 left, Kyrie Irving tossed an alley-oop off the backboard to James for a dunk, giving the Cavaliers their largest lead, 77-57. Fans here were cheering and smiling and putting Wednesday night’s crushing World Series loss by the Indians behind them.

But the Celtics kept Cleveland within reach, and then in the fourth quarter made a final push.

A 3-pointer from the left arc by Bradley with 7:41 left capped a 9-0 run and pulled the Celtics within 104-97. Boston twice sliced the deficit to 5 points, but that was as close as it could get.

“Because of how hard we play, we’re always going to be in games,’’ Thomas said. “We’re one of the hardest-working teams . . . We just have to sew up some things.’’

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.