The incident occurred during a heated training session on Tuesday after shoving and pushing resulted in a "cheap shot" to Mirotic's face, according to the National Basketball Association website The Vertical.
The Bulls didn't provide a timetable for Mirotic's return, but a source told KC Johnson of the Tribune that he could be sidelined for as many as three weeks.
The Bulls are now evaluating disciplinary action against Portis for the incident that was witnessed by general manager Gar Forman.
The Bulls, who open their season against the Toronto Raptors on Thursday, confirmed the incident in a widely reported statement.
This season will be out giving the younger guys chances to play, with the Bulls maybe even being the worst team in the National Basketball Association.
Mirotic underwent tests and sustained a facial fracture from the fight, according to ESPN. The 22-year-old averaged 6.8 PPG last season.
Mirotic was set to begin his fourth season as a member of the Bulls, having signed a two-year, $27 million deal this offseason to stay in Chicago.
Bobby Portis was drafted by the Bulls in the first round in 2015.