Mirotic is out indefinitely after suffering fractures to his upper jaw and a concussion from the incident.
According to reports, the players had been talking trash to one another during practice and it eventually escalated into a physical encounter, per ESPN’s Adrian Wojnarowski.
Mirotic reportedly charged at Portis twice before Portis threw a punch and connected with Mirotic’s face, dropping him to the floor where he laid for several minutes before getting up.
Mirotic was taken to the hospital and released later Tuesday and surgery is likely for him according to the team.
Bulls general manager Gar Forman was at the practice and witnessed the altercation.
Portis has a telephone call with NBA officials in New York on Wednesday to discuss the incident and the Bulls are working with the NBA on possible disciplinary action.