Butler drove into the paint with about three minutes remaining in the third quarter and missed a short jumper. After grabbing the rebound, he pivoted and planted hard on his right foot, before collapsing to the floor in pain, holding his knee.
Teammates, coaches and athletic trainers tended to Butler for at least three minutes before he was carried off the court by two teammates. He was unable to put any weight on the leg as he was helped to the locker room.
Butler was seen hopping on one foot in the Toyota Center hallway as he headed to get an X-ray. Minnesota coach Tom Thibodeau said early evaluations by the team's medical staff were inconclusive and that Butler will undergo an MRI on Saturday when the team gets back to Minnesota.
"You've got to wait for the doctors to do their thing," Thibodeau said. "Until they do the MRI, it's speculating. We'll know more tomorrow. I don't want to speculate on what it might be, but we'll just hope for the best."
Butler did not speak to reporters after the game but was in high spirits, according to multiple teammates. He also was visited in the training room by Rockets guard James Harden.
"His first reaction was, 'I'm OK, I'll be OK,'" Wolves forward Taj Gibson said. "But he was smiling. That was a big thing, and it was a huge relief for everybody in this locker room. He's a warrior, so we've just got to hold it down for him.''
Butler, 28, leads the Timberwolves with 22.2 points per game. He also is tops in the league in minutes played per game at 37.1, but he drew criticism for choosing to sit out the All-Star Game.
Butler told ESPN's Cassidy Hubbarth before Friday's game that he'd rather sit out the NBA showcase than miss time with Minnesota.
"I'm not going to say that I want to cut back my minutes [with the Wolves], but when I have an opportunity [to rest], I will," Butler said.
The Associated Press contributed to this report.