Mirotic is expected to miss at least four to six weeks.
John Paxson, the Bulls' vice president of basketball operations, acknowledged that both players' tempers escalated during practice Tuesday.
"Both players owned responsibility in the incident itself," Paxson said Wednesday. "But only one player threw a punch. And that punch connected and for us that is inexcusable. It's not who we are. It disappoints us in terms of what happened, and because of that we've determined that we're going to suspend Bobby for eight games."
The incident is a blow to a Bulls franchise in the midst of a rebuild. Mirotic was expected to start in coach Fred Hoiberg's revamped lineup, having beaten out Portis. Now, rookie Lauri Markkanen will get his first career start during the season opener Thursday against the Toronto Raptors.
"Bobby Portis is not a bad person," Paxson said. "He's a good kid. He's a competitive kid, but in this instance he made a mistake. And as we all know when you make mistakes you got to suffer the consequences. ...
"This is a team suspension. We did get counsel and advice from the league because this is really unprecedented in a practice situation. There's altercations a lot of times, but the result of Niko's injuries are why we've taken the position that we have."
Paxson confirmed that the conflict between both players escalated throughout a physical practice, with Bulls assistant coach Randy Brown stepping in at one point to calm the rising emotions. Paxson said it would be inaccurate to characterize Portis' punch as a "sucker punch."
Mirotic and Portis have spent two full seasons competing against one another for playing time.
"These type of things do happen," Paxson said. "I've been a part of them. I've seen them. Positionally, it can happen. I used to get into these little scuffles with B.J. [Armstrong] when we played, but you can't cross the line. And Bobby crossed the line. And again, the result of the contact and what it's done to Niko is really the thing that has a lot of us sitting here feeling really disappointed in this today."
Hoiberg said he was "very disappointed." He said he went to visit Mirotic in the hospital on Tuesday before he had been released and had spoken to a remorseful Portis in the aftermath of the fight.
"My job is to not let this moment derail us," Hoiberg said. "My job is to get these guys prepared to go out and fight and play as a group. And I'm confident that our guys will do that."
Veteran center Robin Lopez made it clear that the rest of the Bulls' players felt there was blame on both sides of the altercation.
"We talked with Bobby," Lopez said. "Obviously he's regretful about what he did. We talked about it as a team afterwards. Everything was still so fresh. We as a team feel there's blame on both sides. When it comes down to it the punch was inexcusable."