Pomeranz was placed on the 10-day injured list with a left shoulder strain, retroactive to Aug. 19.
Manager Jayce Tingler revealed Thursday that Pomeranz was day-to-day with shoulder tightness, but the injury is apparently more serious than anticipated. It's unclear whether the southpaw will be able to return from the injured list when he's first eligible. Pomeranz had served as the Padres' primary closer recently, so Emilio Pagan could be in line for ninth-inning duties while Pomeranz is sidelined. Pagan has also been struggling, however, so Jose Castillo could also get some save chances. Right-hander Luis Perdomo was recalled from the team's alternate training site in a corresponding move.