Peavy won't pitch against the Diamondbacks due to back stiffness, while his fellow right-hander Cain has been experiencing right forearm tightness and underwent an MRI on Monday.
Peavy was expected to make his season debut in the second game of the three-game series against their NL West rivals, but that start will go to Ryan Vogelsong instead.
"It's been better but we didn't want to just throw him out there," Giants manager Bruce Bochy said. "It may keep him down for five to six days. He may be ready in San Diego. We'll see where he's at then and if he's ready we'll plug him in there."
Peavy pitched well in 12 starts after joining the Giants last season near the trade deadline but struggled in the postseason and wasn't that sharp in his final two spring starts.
Bochy said Cain mentioned his issue Friday after his last spring start, in which he threw four scoreless innings against the Athletics.
"It's iffy," Bochy said of Cain's chances of making his scheduled start on Wednesday. "If you're asking me, we've got to be concerned."
Cain underwent surgery on his right elbow last August to remove bone chips.
If Cain can't make his start on Wednesday, Bochy might go with Tim Lincecum or a minor leaguer.