Reds manager Terry Francona said Friday that an MRI on Santillan's left oblique showed a significant strain, Charlie Goldsmith of Fox 19 Now Cincinnati reports.
Santillan -- who has notched each of the Reds' last three saves -- was placed on the 15-day injured list earlier Friday, and he appears to be facing a prolonged absence. Emilio Pagan (hamstring) is expected back from the IL soon and could immediately retake his closer job. Until then, Pierce Johnson and/or Tejay Antone could get a look in the ninth inning.