"I feel bad for Sean," Reds manager Bryan Price said Saturday. "He's worked so hard to try and work his way through this recurring shoulder issue. It's been a challenge for some time now."
Shoulder injuries have limited Marshall to 31 games since 2013, but he has not pitched at all this season after having surgery June 24 to repair a torn labrum. He also endured shoulder tendinitis in 2013.
Marshall, 32, led the Majors with 231 appearances over a three-season span from 2010-12. He was acquired by Cincinnati from the Cubs on Dec. 23, 2011, in a deal that sent three players to the Cubs, including left-handed starting pitcher Travis Wood.