CLEVELAND -- The Boston Celtics entered this week knowing a pair of national TV games against undefeated Eastern Conference rivals might provide a bit of an early-season litmus test. But then Al Horford got concussed on Monday, Jae Crowder sprained an ankle on Wednesday and rookie Jaylen Brown was asked to defend LeBron James in his first NBA start on Thursday.

The Celtics showed some grit both in pulling out Wednesday's win over the late-charging Chicago Bulls and making the Cleveland Cavaliers work to finish off a 128-122 triumph on Thursday night at Quicken Loans Arena. Still, it didn't sit well with the players that injuries cost Boston a chance to put its best foot forward and truly assert itself as a legitimate contender in the East.

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"It's very disappointing," All-Star point guard Isaiah Thomas said after the shorthanded Celtics kept fighting despite falling behind the Cavaliers by as much as 20 in the first half. "There’s no excuses. We played a game we felt we could have won, and we only lost by six. But we have to do a lot better in certain situations. Especially against teams like this, we have to play better."

Five games into the season, it's hard to gauge exactly where Boston stands. The Celtics have yet to play with anything close to a full roster. They started the season with Marcus Smart (ankle) and Kelly Olynyk (shoulder) on the shelf. Smart returned on Wednesday, but starters Horford and Crowder were out. Both Horford and Olynyk should be back soon, but coach Brad Stevens said Crowder likely will miss at least two weeks, which would sideline him for at least six games.

Health woes could be an issue as the Celtics play 20 of their first 34 games on the road. Thursday's game started a stretch in which eight of 12 will be played away from home, including a pair of road games on the second night of back-to-backs.

In addition to matching up with LeBron James much of the night, Celtics rookie Jaylen Brown had a career-high 19 points Thursday. Ken Blaze/USA TODAY Sports

If there's any sort of silver lining to Crowder's absence, it's that Brown is going to get thrown to the wolves -- in a good way. To keep Smart with the second unit, Stevens elevated Brown, the No. 3 pick in June's draft, to Crowder's starting spot Thursday. That meant matching him up early and often with James.

Brown didn't just embrace the challenge -- he held his own, including forcing James into an early miss. Offensively, Brown was aggressive going at the basket. Five of his eight field goals came at the rim, and that included a second-quarter drive in which he got James leaning with a pump fake at the 3-point arc, then won a foot race to the charge circle before delivering a two-handed jam.

Brown finished with a career-high 19 points on 8-of-16 shooting with five rebounds, three steals, two assists and a block over 35 minutes, 26 seconds of floor time. Brown was a team-best plus-15 in plus/minus; no regular was even close to that number.

After the final buzzer, James sought out Brown and offered some words of advice.

"I'm not going to tell you what I told him. If he wants to bring it up, that's fine," said James, who finished with 30 points, 12 assists and 7 rebounds.

"But I think he's a really good talent and that's why he was drafted so high. He's a strong kid. You can see he knows how to play the game."

Then James took the opportunity to kick the Brooklyn Nets while they are down.

"Boston definitely is enjoying all the picks that they're getting from the Nets the last few years," James said.

Brown said before Thursday's game that he was excited about the challenge of defending James because "LeBron’s the best player in the world right now. So if I want to ever get to that level, you’ve got to go through him." After the game, Brown reported no nerves, though he admitted he did have to pinch himself a bit.

"It’s time to play basketball. It’s not a game, it’s not a movie. This is real life," he said. "You’re here in the NBA, your feet are set, now it’s time to make a name."

Brown might have accomplished that task. His name briefly trended on Twitter and it wasn't just because of the dunk on James (though that immediately becomes Brown's signature highlight). Brown hit a trio of 3-pointers, including two in the fourth quarter when Boston trimmed Cleveland's lead as low as five before James helped seal the victory.

So what exactly did James say to Brown after the game?

"He just said he’ll see me down the road," Brown said. "He said good job, and he said some other things that I ain’t gonna mention."

But the fact that James sought him out certainly resonated with the 20-year-old rookie.

"I guess it means a lot," Brown said. "I respect LeBron, not just on the floor but off, what he does off the floor donating to inner-city kids, how he handles himself. He's never been in trouble with the law or anything like that. He’s just a great role model for somebody like me, and I watched him growing up and he’s kind of shaped how I am now. Just, off the floor, how he moves so, just how I respect him on the floor -- I respect him even more off. He’s a great role model.

"At the end of the day, I’m a competitor, and I think I’m more competitive than a lot of people in this league. I had a focus. I wasn’t nervous, I wasn’t starstruck or anything. I came out and had a job to do and tried to do it to the best of my ability."

When asked if Crowder's injury situation might ultimately be a good thing for his development, Brown wasn't so quick to agree.

"It is an opportunity but, at the same time, Jae Crowder is going to have a speedy recovery and come back. We’re going to have a streak of wins and we’re going to do what we’re supposed to do. Hopefully he gets back nice and quick, and hopefully then we’ll be clocking on all cylinders. But while he’s down, I’m going to be ready and do the best I can."