Thomas, who finished with four points in 20 minutes, suffered a badly bruised lower back while being fouled by Dwyane Wade in a game against the Heat on March 9. Swelling persisted, limiting his ability to return to basketball activities, but he felt comfortable enough to get back on the floor Wednesday in a key matchup between teams battling for a final playoff spot in the Eastern Conference.
Celtics coach Brad Stevens felt Thomas was searching for his game.
"Hopefully that was just a rhythm thing and not how he's feeling," Stevens said after the loss. "That's what I want to know."
"I'm as good as I'm going to be," Thomas said before Wednesday's game. "I'm not 100 percent, I'll work through it."
Later he added: "[The back has] improved. I'm able to play. It's improved a lot from that day when it happened and the day after. It's still going to be a process and if I was trying to be 100 percent, I probably wouldn't play the rest of the year."
Thomas admitted he's worried a bit about another hard fall or getting hit on the injury, but thinks adrenaline will help carry him, especially with his wind after only recently resuming full-court activities this week.
"My bruise -- if I showed you the picture, it would be nasty," said Thomas. "It was one of the worst bruises I've ever seen. I couldn't sleep, I couldn't sit down for three or four days after. It was horrible, but I'm good now. I'm good as I'm going to be. Just happy to be back playing, back moving."
Stevens said he'll determine how much Thomas plays by how he looks on the court. He is under no minute restriction.
"That's a hard balance, because you want him to get his feel back," said Stevens. "We also want to play well and I think he'll play well. I think he's excited about playing, but if he gets hit there, it is to be determined how he reacts."
Thomas was asked how he felt about the hard foul by Wade that injured him earlier this month. Wade glared at him on the floor after the contact on a strong drive to the basket in a game that Boston won.
"The only thing I didn't like about it is how he looked at me after," said Thomas. "You can't control that. He tried to block the shot. Only thing I thought about when I watched the film was how he looked at me after. Hopefully we get the win and I'll get a little revenge."
The Celtics went 5-3 during Thomas' absence. Thomas had been averaging a team-best 21.4 points over 27.6 minutes per game in 10 appearances since being acquired at February's trade deadline. The Celtics have missed his scoring prowess, especially in the fourth quarter when he had emerged as their go-to option in close games.
Information from The Associated Press was used in this report.