Davis, who was rehabbing from elbow surgery, pitched for the Triple-A Iowa Cubs on Tuesday. The 35-year-old will take the rotation spot of struggling James Russell.
Davis was 1-4 with a 7.51 ERA in eight games with the Milwaukee Brewers last season. Aside from the elbow surgery, Davis was also diagnosed with a mild case of pericarditis, an inflammation of the membrane surrounding the heart, on May 15. He only made one appearance after May 10. Davis also missed part of the 2008 season while he was being treated for thyroid cancer.
Signed to a minor league contract on April 12, Davis' base pay will be $900,000.
ESPNChicago.com's Bruce Levine contributed to this report.