A person with direct knowledge of the initial diagnosis told ESPNDallas.com's Tim MacMahon that the symptoms indicated Butler suffered at least a partial tear of the patellar tendon in his right knee.
If it is a partial tear, Butler could be out two months. However, if it's a full tear, he would miss the rest of the season. The Mavericks won't be certain about the extent of the injury until they get the results of an MRI that will be taken after Butler returns to Dallas on Sunday.
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.
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.
Nowitzki has a sprained right knee. He is day to day, and Dallas is 1-3 without him.
Butler, a small forward, is the team's third-leading scorer at 15.0 points per game. He plays an average of 30 minutes. The injury 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. He scored 22 points in Dallas' 104-95 win.
Rookie Dominique Jones, the Mavericks' first-round pick, joined the team in Cleveland for Sunday's game. Jones had been playing for the Texas Legends, the Mavericks' D-League affiliate, averaging 18.2 points, 5.2 rebounds and 5.0 assists in 10 games.
Information from ESPNDallas.com's Tim MacMahon and The Associated Press was used in this report.