The club began the year with just four starters in the rotation thanks to a pair of off-days early in the schedule. Come Saturday's game at San Diego, though, the D-backs will need a fifth starter.
The candidates at Triple-A Reno include Kris Benson, Billy Buckner and Kevin Mulvey.
Buckner and Mulvey were outpitched for the fourth spot in the rotation by Rodrigo Lopez this spring and Benson was signed after spring games had gotten under way.
"Well before Saturday," D-backs manager A.J. Hinch said when asked on when he was going to choose a fourth guy. "Probably in the next day or so. We need to game plan a little bit, but I don't really care for the Padres to know quite yet."
Mulvey was set to pitch Tuesday night. Benson pitched Monday and allowed two unearned runs on four hits over five innings. Buckner, meanwhile, allowed seven runs on 10 hits in five innings on Sunday.
While Benson might have performance working in his favor, the thing he does not have is a spot on the 40-man roster. In order to add him, the D-backs would have to move someone off. One possibility would be to place Brandon Webb on the 60-day disabled list which would take him off the 40-man.