The Sox announced X-rays were negative and Peavy underwent an MRI on Wednesday before the Sox completed their series against the Seattle Mariners. Manager Robin Ventura said the team will wait on the MRI results before making a decision on Peavy.
"You get all the tests you can, see what it is and you go from there," Ventura told reporters. "Until you get that information the only thing you can really do is line up Hector in that spot before you go on from there."
Peavy allowed six runs on seven hits over 2 1/3 innings on Tuesday in a second consecutive subpar outing. He gave up six earned runs over four innings against the Chicago Cubs on Thursday.
Santiago is 1-2 with a 2.96 ERA in five starts this season. His last start came against the Boston Red Sox on May 22 when he allowed two earned runs with nine strikeouts in six innings, but took the loss in a 6-2 White Sox defeat.