Mendoza was scratched from his scheduled start on Wednesday night against the White Sox. The Royals didn't play on Monday so left-hander Bruce Chen can move up a day and take Mendoza's spot, with right-hander Jeremy Guthrie opposing Chicago on Thursday night.
"[Mendoza] came in today and was feeling horrible," Royals manager Ned Yost said. "His throat was all infected. We think it's strep throat. We sent him to the doctor."
The rotation for the weekend series against Cleveland at Kauffman Stadium remained uncertain, pending Mendoza's health and ability to pitch.
Mendoza, left-hander Will Smith and just-arrived Jake Odorizzi are in the mix. It's possible that Odorizzi, promoted from Triple-A Omaha, will make his Major League debut on Saturday night or Sunday against the Indians.
It all depends on Mendoza's availability.
"We may just skip his whole turn, I don't know. We've got to wait and see," Yost said.
Mendoza's illness will prevent him from going head to head against White Sox ace Chris Sale for the fourth time this season. Sale topped Mendoza twice on July 15 and Aug. 6, with Mendoza beating Sale on Aug. 17.
It sets up a Sale-Chen rematch of Sept. 8 in Chicago. Sale beat Chen in that game.