Right elbow tightness forced Marcum to on the disabled list on June 23, retroactive June 15.
Marcum's bullpen session will likely determine how quickly the rest of his rehab goes. Manager Ron Roenicke said Marcum won't be back next week.
"Big day for Shaun tomorrow," Roenicke said. "That'll be good to see where he is. He's felt really good playing catch, and it'll be nice to get him back on the mound."