Marmol suffered the injury last night, pitching the seventh inning of the Cubs' 8-7 loss to the Milwaukee Brewers. He returned to Chicago for an MRI.
"He was really sore when he was walking out last night, and it seemed to actually be getting worse," Cubs manager Dale Sveum said.
Marmol has had a disappointing season thus far -- he's 0-1 with a 6.35 ERA in 15 games. He was relieved of his closer duties earlier this month.