Butler drove into the paint with about three minutes remaining in the third quarter, missed a short jumper, but then clawed the rebound. He dished to a teammate, then crumpled to the floor in pain, holding his right knee.
Teammates, coaches and athletic trainers tended to Butler, who had to be helped off the court. He was unable to put any weight on the leg as he was helped to the locker room, where he was evaluated.
Butler was seen hopping on one foot in the Toyota Center hallway as he headed to get an X-ray. He is scheduled to have an MRI on Saturday when the team gets back to Minnesota.
"Just hopeful that it's not bad," said Wolves coach Tom Thibodeau.
Butler, 28, leads the Timberwolves with 22.4 points per game. He also is tops in the league in minutes played per game at 37.3, but he drew criticism for choosing to sit out the All-Star Game.
Butler told ESPN's Cassidy Hubbarth prior to 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.