4:18PM PT -- The Bulls released a statement addressing the incident.

"Chicago Bulls forwards Bobby Portis and Nikola Mirotic had a physical altercation during today's practice. As a result of the incident, Mirotic suffered a concussion and maxillary fractures. Surgery is likely required. Mirotic is out indefinitely."

Bulls forward Bobby Portis socked teammate Nikola Mirotic and broke his face and gave him a concussion at practice today.

Portis and Mirotic reportedly got into a shoving match ... that ended with Bobby cheap-shotting Nikola ... The Vertical reported.

Mirotic was taken to the hospital with multiple broken bones in his face. He was also evaluated for a concussion. The report says N.M. has since been released from the hospital.

Mirotic was expected to begin the season as the team's starting PF, ahead of Portis.