The Reds selected Sanmartin's contract from Triple-A Louisville ahead of Monday's game against the Pirates.
Cincinnati designated catcher Beau Taylor for assignment and placed Luis Castillo (personal) on the family medical emergency list to clear room for Sanmartin on the 40-man roster and 28-man active roster, respectively. Sanmartin will immediately slot into the rotation Monday, and he could line up for a two-start week with the status of Castillo and Wade Miley (neck) in question for next weekend's three-game series in Pittsburgh. Sanmartin, a 25-year-old lefty, submitted a 3.94 ERA, 1.25 WHIP and 89:23 K:BB across 82.1 innings at Louisville this season.