The Reds placed Santillan on the 15-day injured list Tuesday with a left hip labral tear.
Santillan's hip had been bothering him for the last couple weeks, per Charlie Goldsmith of Fox 19 Now Cincinnati, and manager Terry Francona said the reliever will likely have surgery later this week. He'll finish the campaign with a 6.42 ERA and 36:28 K:BB over 40.2 frames.