Perez, who appeared to wince in pain after swinging at strike three, has dealt with tightness on either side of his rib cage this season. Perez is batting .278 with 21 home runs. He was replaced by Drew Butera.
"It's the right side, but it's a different spot," Yost said. "It's a little more concerning."
Wilson Alexander is a reporter for MLB.com based in Kansas City. This story was not subject to the approval of Major League Baseball or its clubs.