Peavy, 31, is 3-1 with a 3.38 ERA in five starts this season. He has missed time with groin and shoulder problems in recent years, including a detached lat muscle in 2010 that required a unique surgery. Peavy hasn't had any back trouble in the past.
Santiago, 25, was scheduled to take the rotation spot of the injured Gavin Floyd this weekend at Kansas City. The team will need to come up with another starter to cover that game, assuming Peavy is unable to go. Santiago owns a 2.51 ERA with 11 strikeouts in 14 1/3 relief innings this year.
The White Sox are already without Floyd and left-hander John Danks (shoulder surgery).