Per Colin Dunlap at the Post-Gazette, Pedro Alvarez could be headed to the disabled list because of his quadriceps, while Pedro Ciriaco will take his place on the roster.
He doesn't say who would take Alvarez's place in the lineup, but I would assume we're in for a steady diet of Brandon Wood, with a little help from Steve Pearce. He also doesn't say how long Pedro could be sidelined.
Tough break. El Toro seemed to be heating up a little. Then again, maybe the time off will give him a chance to clear his head and tweak his mechanics.
UPDATE by Charlie: Colin Dunlap tweets that Alvarez is indeed headed to the DL.