A person with direct knowledge said the initial diagnosis is a torn patellar tendon. The person spoke to The Associated Press on condition of anonymity because the team was awaiting MRI results before making any announcements.
Butler went down during a loss at Milwaukee on Saturday night. He was coming off a season-high, 30-point performance the previous game while picking up the offensive slack because of an injury to leading scorer Dirk Nowitzki.
Dallas coach Rick Carlisle had no new information about the injury before Sunday's game at Cleveland. Carlisle said Butler was flying from Milwaukee to Dallas Sunday evening. Carlisle wasn't sure when the MRI will take place.
"Anyone who speculates beyond that should be very careful in what they do because it could have ramifications," Carlisle said.
Nowitzki, who has a sprained knee, was out again Sunday. He is day-to-day, and Dallas is 0-3 without him.
Butler, a small forward, is the team's third-leading scorer at 15 points per game. He plays an average of 30 minutes. A long absence could be especially costly to him because he will be a free agent after this season.
Shawn Marion, who missed the Bucks game with a thigh problem, returned to action Sunday.
Dallas also recalled rookie forward Dominique Jones from the Texas Legends of the NBA Development League.