The Bulls are scheduled to open their season Thursday in Toronto against the Raptors.
According to The Vertical's Shams Charania, Mirotic and Portis got into a shoving altercation during practice before Portis threw a punch, connecting with Mirotic's face.
"Bobby Portis is not a bad person", Paxson said to reporters on Wednesday after practice, via Sean Highkin.
The team's statement also confirmed Mirotic suffered "a concussion and maxillary fractures" and will likely need surgery.More news: Latest Windows 10 update rolled out
NBC Sports Chicago's Vincent Goodwill added that this incident has been brewingbetween the two forwards for three years, and that for what it's worth, Mirotic was the instigator.
"We understand where we are as an organization", vice president of basketball operations John Paxson said. But in this incident, he made a mistake.
Paxson said that the Bulls got counsel from the league on how to handle the situation. The Bulls are evaluating disciplinary action. Portis will be eligible to return for the Bulls' ninth game of the season, which is coincidentally also at Toronto on November 7.
Once Portis comes back from his inevitable suspension, our models project him to average 8.9 points and 5.4 rebounds over 22.4 minutes per game. He signed a two-year deal with $12.5 million guaranteed this season to stay in Chicago.