The Reds placed Santillan on the 15-day injured list Tuesday with a left hip labral tear.
It's unclear how long Santillan's hip has been bothering him, but he really struggled in 10 appearances since returning from an oblique strain last month, logging a 10.24 ERA and 8:14 K:BB over 9.2 innings. Santillan's season is now over, but it's unclear whether he will need surgery to repair the hip. He'll finish the campaign with a 6.42 ERA and 36:28 K:BB over 40.2 frames.