"It was a small setback," manager Dusty Baker said. "Any setback takes you a week or so from where you were."
The Reds have also been without right-handed setup man Jonathan Broxton, who has been on the DL since June 15 with a sore elbow. Broxton, however, is much closer to a return than Marshall.
Marshall owns a 2.57 ERA and 0.86 WHIP (walks and hits per inning pitched) out of Cincinnati's bullpen, and Baker said the club needs its relievers to return as quickly as possible.
"The remaining games are getting fewer and fewer," Baker said.
Andrew Owens is an associate reporter for MLB.com. Follow him on Twitter @OwensAndrew. This story was not subject to the approval of Major League Baseball or its clubs.