While warming up before Wednesday's 5-2 win over Texas, Peavy completed his usual yoga stretch and bent over to pick up his glove when he felt a pain in his back so sharp that he went to his knees.
"I've never had anything like this happen,'' Peavy said after extended treatment in the training room Thursday. "It is completely new to me. And completely unexpected.''
Peavy in five starts is 2-1 with 3.38 ERA.
"Jake had a little bit of it yesterday and we were just waiting to see if he felt better today,'' said White Sox manager Robin Ventura. "There's no reason to send him out there tonight. But it's not an injury that should keep him out for more than a few days.''
Ventura said Dylan Axelrod, originally slated to pitch on Saturday, becomes the scheduled starter in Friday's opener of a three-game series in Kansas City. Santiago was originally scheduled to start that game.
Peavy said his return to the rotation is uncertain at this point. But he expects it to be longer than a day or two.
"Hopefully, it will be back to normal soon,'' said Peavy, who is 123-94 in his 11-year career. "We've got a great medical staff. We still have to put a game plan together.
"I hate doing this to the guys, not going out there for them. If there was any way to pitch, I would have been out there."
"It's not fun,'' said Ventura of the rash of injuries. "Every day you go through the list and figure it out.''