The Brewers made the move Friday.
Marcum has been on the disabled list since June 23 because of tightness in his right elbow. He made three rehabilitation starts in the minors and Milwaukee manager Ron Roenicke said Marcum would be on a pitch count of approximately 90 on Saturday.
Marcum is 5-3 with a 3.39 ERA in his second year with Milwaukee and can become a free agent after the season. He will get two starts before Aug. 31, when postseason rosters are set.
Roenicke openly suggested Friday that Marcum would be an attractive trade candidate if he pitches well. Roenicke also referenced reliever Francisco Rodriguez in similar fashion.