Garza lasted just four innings in his return, giving up four runs in an 8-2 loss.
"Physically I felt fine," Garza said. "I was a little off. That's not what I wanted. I wanted to come with five or six strong (innings) and do what I usually do. I felt strong. I think it was just mainly 17 days of not facing a hitter."
To make room for Garza, the Cubs optioned reliever Scott Maine to Triple-A Iowa.
Garza went on the DL on May 21 after complaining of right elbow pain during his side session on May 19. An MRI revealed he had a right elbow contusion.
Garza, who threw a second side session on Friday, entered Monday leading all starters in baseball with 10.99 strikeouts per nine innings. He was 2-4 with a 3.72 ERA in his first season with the Cubs.
Maine had a 10.29 ERA in seven games with the Cubs.
Information from The Associated Press was used in this report.