Zambrano, who was pitching in Dunedin, Fla., has been on the disabled list since May 4 because of a strained left hamstring. He walked the first two batters he faced.
"No problems, it's 100 percent," Zambrano told reporters in Florida. "I just had a little problem with my release point. It's been two weeks since my last outing, so between now and Friday I have a lot of time to correct my release point."
Zambrano threw 67 pitches, 42 for strikes, in the rehab outing.
The right-hander is supposed to rejoin the Cubs in St. Louis this week, and if he passes all the tests, would return to the rotation Friday in San Diego.
The Cubs will make a decision on their roster Friday. One option is to put Sean Marshall in the bullpen to give Lou Piniella another left-hander and keep Randy Wells in the rotation. Wells has thrown 11 scoreless innings in his two starts.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.