Chicago Bulls forward Nikola Mirotic is out indefinitely after getting into a fight with a team-mate during practice, the team said on Tuesday.
Mirotic, 26, suffered a concussion and facial fractures after a physical altercation with fellow forward Bobby Portis.
The Bulls, who open their season against the Toronto Raptors on Thursday, confirmed the incident in a widely reported statement.
"Chicago Bulls forwards Bobby Portis and Nikola Mirotic had a physical altercation during today's practice," the statement read.
"As a result of the incident, Mirotic suffered a concussion and maxillary fractures. Surgery is likely required. Mirotic is out indefinitely.
"The Bulls are evaluating disciplinary action. An update will be provided when applicable."
Mirotic signed a two-year, $27million contract to return to the team in the offseason.