An MRI before Chicago played Milwaukee on Tuesday night revealed the injury. Rizzo said he will miss an extended amount of time.
Rizzo was hurt on Aug. 26 and has been out of the lineup for a week.
The 25-year-old Rizzo is hitting .278 with 30 home runs and 71 RBIs.
''Certain things I do I feel it. Twisting is when I feel it most. When I swing, it hurts. It's kind of relieving that the MRI showed something, so I'm not going crazy anymore,'' he said. ''Hopefully in 10 days, I'll feel a lot better and start up again.''
''If I'm shut down, I'm shut down. My main concern is being full strength in the offseason, getting ready for next year,'' he said. ''Obviously, I want to come back and play. It's not fun sitting around and watching.''