Cubs manager Lou Piniella said Zambrano had "appendicitis-type symptoms," but the team later reported that tests were negative and the pitcher had problems with abdominal pain on his right side.
Zambrano reported the problem to the Cubs' athletic trainers shortly before Thursday's game against the Dodgers started, and he was taken to Northwestern Memorial Hospital to be examined. There was no word on his availability this weekend.
Zambrano was to have been back in the bullpen on Thursday. He is prepping for a return to the Cubs' rotation next week, which was expected to be during the Pirates series from Monday to Wednesday.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.