Robles was placed on the 15-day injured list with back spasms Saturday, retroactive to May 25.
Robles hasn't pitched in nearly a week, and his lack of appearances was apparently due to back spasms which will keep him sidelined until at least early June. The right-hander has converted two of five save chances this year while posting a 2.65 ERA and 1.06 WHIP in 17 innings over 16 appearances. Most of Boston's other late relievers have struggled recently, but John Schreiber, Matt Barnes and Matt Strahm will likely be in the mix for saves while Robles is unavailable.