Zambrano was seen stretching his back in the first inning and then came out of the game after giving up a walk, a bunt single an RBI bloop single to the first three batters of the second inning.
He was replaced by Marcos Mateo who was able to get out of the inning without further damage.
Zambrano is 6-4 with a 4.34 ERA this season. He had allowed two hits and an unearned run, striking out one when he left. He did have an error that led to the one run that scored.
UPDATE: Zambrano went to the hospital to have an MRI, but the Cubs don't believe it's anything serious (MLB.com via Twitter).For more baseball news, rumors and analysis, follow @cbssportsmlb on Twitter or subscribe to the RSS feed.