"It's a mid-back strain," Cubs general manager Jim Hendry told ESPNChicago. "It's in the back side near his oblique. He did it in the pen the other night in his side session. Trainer Mark O'Neal told me it wasn't really bad, but we don't want to take any chances with Rich. We want to get it right for him."
The side session in question came Wednesday in St. Louis. The Cubs placed him on the 15-day DL, retroactive to Sunday, when he last started.