From Will Carroll's "Under the Knife" column at Baseball Prospectus (subscription):

The most worrisome aspects of Zambrano's start (on Tuesday) were the arm angle and the quick drop in velocity. By the third inning, he was down from 93 to 90, and by the fourth and fifth he was exerting more effort just to stay in the 90 range. The extra time off might have made him stronger in the early going, but it didn't last time out...

On Thursday, (Zambrano) did receive another cortisone shot and had scheduled tests done that apparently showed he's having another bout of rotator cuff tendonitis. Given his results since last time, I'm not sure that this is going to be comforting news to Cubs fans, especially if he comes back next week with his lower arm slot.