It appears -- in the short term, at least -- that slot will either be filled by left-hander Robbie Erlin or right-hander Tim Stauffer. Righty Burch Smith would seem to be out of the mix after he pitched five innings for Triple-A Tucson on Saturday.
The 22-year-old Erlin is 8-3 with a 5.07 ERA for Tucson. He has made six appearances for the big league club this season, and is 1-2 with a 7.15 ERA.
The primary reason for the timing of the Volquez move was to allow the club a chance to evaluate some of its young pitching options at the big league level, meaning both Smith and Erlin will likely get a look once rosters expand Sept. 1.
Still, the veteran long-reliever Stauffer remains the only option on the active roster. Black said Stauffer was unavailable for Sunday's game against the Cubs, but that was the result of his 4 1/3 innings out of the 'pen on Friday.
AJ Cassavell is a reporter for MLB.com. Follow him on Twitter @ajcassavell. This story was not subject to the approval of Major League Baseball or its clubs.