Porter was evaluated Monday in Green Bay, Wisconsin, by Dr. Bob Anderson, who confirmed the bone injury.
The injury was initially diagnosed as a bruise, but the small fracture became clear after repeated imaging tests over the past five weeks, the team said.
According to the Bulls, Porter "will continue his current period of immobilization and progress as tolerated over the next four weeks when he will be reassessed."
Porter suffered the injury against the Hawks on Nov. 6. He is averaging 11.2 points in nine games this season.
The Associated Press contributed to this report.