The Bulls say a second MRI on Tuesday revealed the bruise, something “not apparent” on the first MRI.
“He had a second scan, and something showed up that didn’t show up on the first scan,” Bulls coach Jim Boylen told NBC Sports Chicago. “That’s tough, tough for him, feel bad for him. But we’ll treat him and move on.”
Chandler Hutchison had been filling in Porter’s spot in the rotation, but he is out indefinitely now with sore shins.
Porter, averaging 11.2 points in nine games, has been sidelined since he sprained his left foot at Atlanta on Nov. 6.
The Bulls were 4-10 with three straight losses heading into Wednesday’s game against Detroit.