Santillan was optioned to Triple-A Louisville on Friday, Charlie Goldsmith of The Cincinnati Enquirer reports.
The right-hander covered 3.2 scoreless frames in his past three outings, but he'll head back to Triple-A with Michael Lorenzen (hamstring) returning from the injured list. Santillan will continue working as a reliever with Louisville and could rejoin Cincinnati's bullpen down the stretch.