James retreated to the locker room with 3:50 remaining in the second quarter after he collided with Boston's Jayson Tatum and was fouled.
Before checking out, James made one of two free throws, giving Cleveland its largest lead of the game at 11 points. The Cavs were outscored by 24 the rest of the way after the collision, losing 107-94 and falling behind 2-0 in the series.
"It was a tough collision," Cavs coach Tyronn Lue said. "Knocked his head sideways. Of course, he came back; but when he came back, I don't think he had the same punch that he had before he left as far as attacking the basket, playing with that force and power we talked about at shootaround. We'll see how he feels."
Asked in the locker room by ESPN after the game if his head was OK, James nodded solemnly and said, "I'm OK."
Tatum's left shoulder hit James' face and jaw.
"I felt like I needed to go back to the locker room, which I did, and kind of recalibrate," James said during the postgame press conference. "It was a tough blow. Obviously incidental. His shoulder hit me right square in my jaw. Just wanted to go back to the back and make sure everything was fine. But I don't think it -- it didn't affect my game after that."
James opened the game by scoring 24 points on 9-for-15 shooting with four rebounds and four assists in 16 minutes. After the Tatum play, he shot 7-for-14 the rest of the way with six more rebounds and eight more assists.
Cavs forward Jeff Green subbed in temporarily for James, who returned with 1:57 remaining in the second quarter.
Despite the injury, James finished with a game-high 42 points as part of a triple-double, which included 10 rebounds and 12 assists. He did have a game-high six turnovers, giving him 13 in the series so far.
Afterward, when asked if he was examined for a concussion after leaving the court, James said he was checked for a variety of things and cleared all the tests, allowing for his return.
"You know, that was big for us," Celtics guard Marcus Smart said of James' setback. "He went out. I don't think Jayson was intentionally trying [to hurt James]. He was trying to make a basketball play; it just happened. He went out, and it allowed us to pick up our pressure even more."