Zambrano has made a shaky start this season, going 1-2 with a 7.45 ERA and the Cubs are hoping the big right-hander can find his form while helping a bullpen that has blown four saves this year.
Zambrano has not pitched in relief since 2002 and is hoping the move is temporary.
"I don't like to be a reliever. I don't want to be a reliever," Zambrano told reporters.
Veteran Ted Lilly will take Zambrano's spot in the starting rotation as he returns from offseason shoulder surgery.
(Reporting by Jahmal Corner in Los Angeles, editing by Ossian Shine)