The right-handed pitcher left his start on June 18 against Tampa Bay because of pain in his right shoulder. Tests showed a minor strain. Zambrano took the loss in that game, allowing five runs -- four earned -- in 6.2 innings.
Zambrano, who also missed a month in 2002 with a right elbow strain, is 8-3 with a 3.13 ERA.
The Cubs also have optioned pitcher Jose Ascanio to Triple-A.