Bobby Portis is paying the price for his brutal punch on Bulls teammate Nikola Mirotic ... as the team has slapped him with an 8-game ban.

Portis laid out Mirotic at practice on Tuesday ... sending him to the hospital with a fractured jaw and a concussion.

Nikola was discharged from the hospital later that day, but will likely need surgery -- forcing him to sit out at least 4 weeks.

The incident went down after the Bulls forwards got into it with trash-talk and shoving. Multiple reports claim Mirotic was the aggressor -- repeatedly charging at Bobby before being put down.

Which is probably why Portis only got 8 games. Could've been a lot worse. 

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