Manager Dale Sveum said the right-hander will make at least one more rehab start for Triple-A Iowa in five days, and then be re-evaluated. Garza is coming back from a strained left lat suffered in mid-February during live batting practice.
The plan Saturday was to have Garza throw 65 to 75 pitches. He needed 30 pitches to get through the first inning.
The Cubs are not going to go with a six-man rotation when Garza returns, which means someone will get bumped. Sveum said it won't be Edwin Jackson, despite his early season struggles.
Jackson entered Saturday's game against the Nationals with an 0-5 record and a 6.39 ERA, while the other four starters rank among the top 15 pitchers in the National League in opponents batting average against. The right-hander fared better against the Nats, allowing two runs in 5 1/3 innings to lower his ERA to 6.02.
Carlos Villanueva has done well as a starter this season and has never had the chance to make 32 starts. His single-season innings high is 125 1/3, set last year.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.