Cincinnati selected Santillan's contract from Triple-A Louisville on Saturday.
The Reds placed starter Carson Spiers (shoulder) on the injured list Saturday and called up Santillan to take Spiers' roster spot, opting to add a reliever with the All-Star break looming. Santillan has logged a 3.38 ERA, 1.40 WHIP and 53:24 K:BB over 40.0 innings in the minors this season. He got into three games with the Reds last year, allowing one run on four hits while posting a 5:1 K:BB over 3.1 frames. He'll like serve in a low-leverage role while with the big-league club.