Cubs medical personnel said tests for appendicitis proved negative, and the team listed him as day to day. He returned home and was not in pain anymore, a baseball source told ESPN The Magazine's Buster Olney later Thursday.
"He's got some pain in his ... where appendicitis is," Piniella said, pointing to his lower-right abdomen. "He was at the hospital all day."
The Cubs beat the Los Angeles Dodgers 1-0 on Thursday.
Zambrano is scheduled to return to the starting rotation during a road series in Pittsburgh, which is from May 31-June 2.
Cubs infielder Jeff Baker, who started at third base on Thursday, also experienced a scary physical ailment. Piniella said Baker couldn't see out of his right eye and had to be taken out of the game.
According to a team spokesperson, Baker was also sent to Northwestern to consult with team specialist John Rosin. Results of those test are still pending.
"It was a little scary," Piniella said. "He had no vision at all out of his right eye."
"We had to get him out of there."
Baker was lifted in the eighth inning after he didn't even make a move on a ball hit to his left.
In addition, Cubs third baseman Aramis Ramirez received a cortisone injection in his swollen left hand, but he could play Friday.
ESPNChicago.com's Bruce Levine contributed to this report.