The Reds placed Sanmartin on the 15-day injured list Tuesday with a left elbow stress reaction.
Sanmartin will be completely shut down for the next 4-to-6 weeks, though he is expected to return before the end of the season. He had struggled to a 7.07 ERA, 1.93 WHIP and 13:10 K:BB through 14 innings of relief this season with Cincinnati.