Mirotic was hospitalized Tuesday following an incident at practice with teammate Bobby Portis, Shams Charania of The Vertical reports. Mirotic is at the hospital undergoing tests, but he's expected to be out for the foreseeable future.
The Bulls open the season Thursday night in Toronto.
Mirotic, 26, signed a two-year, $27 million deal with Chicago before the season after averaging 10.6 points and 5.5 rebounds last season.
Portis may encounter some team-imposed discipline. It's especially bad when the fight is so severe that one lands in the hospital. Portis is entering his third year in the National Basketball Association after being selected with the No. 22 pick in the 2015 draft.
Additionally, NBC Sports Chicago's Vincent Goodwill is reporting that Mirotic was punched in the face by Portis and is also under evaluation for a possible concussion. The 6-foot-11 Portis was expected to come off the bench while Mirotic was to start at Chicago's four spot.