Davis is scheduled to make his season debut Saturday night against the San Francisco Giants.
To make room for Davis, the Cubs optioned right-handed reliever Justin Berg to Iowa. Chicago also took right-handed pitcher Thomas Diamond off its 40-man roster to the Iowa roster.
The 35-year-old Davis went 1-4 with a 7.51 ERA in eight starts for Milwaukee last season.
A year ago, he was diagnosed with an inflammation of the lining surrounding his heart. He made one more start in July before being shut down with elbow problems.
Davis also missed part of the 2008 season while being treated for thyroid cancer.