Runelvys Hernandez is the likeliest candidate to receive a callup from Triple-A Round Rock to start the fifth game. If Roy Oswalt recovers from his recent setback, he will start Friday against the Cubs, followed by Wandy Rodriguez, Brian Moehler and Brandon Backe.
Oswalt left his start on Friday with recurring left hip soreness, and he will be re-evaluated at a later time. He could start the second half on the disabled list.
Cooper said the club is not "locked in" on Hernandez, but "he's probably the guy that will get recalled to do it."
Hernandez was optioned to Round Rock on Tuesday after a difficult outing during which he allowed 10 runs over four innings.
Although Chris Sampson would be a tempting option, Cooper prefers to leave the right-hander in the bullpen, where he's been dominant. Sampson threw five scoreless innings during a spot start last Saturday in Atlanta and he shut out the Pirates in two relief frames four days later in Pittsburgh.
"He's been doing so well, I'm hesitant to do that," Cooper said of putting Sampson back in the rotation. "I don't think it's something we'd consider."
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.