Team sources say the redshirt sophomore has a stomach illness and that Blake Field, a redshirt freshman, will get his first college start.
Field say his first playing time for the Tigers last week and he completed 1-3 passes for 11 yards.
Cox has completed 72-115 passes for 1,072 yards and nine TDs with six TDs for his career. This season he is 50-81 for 715 yards with four interceptions and five TDs.
Field, who played as a senior at North Florida Christian in Tallahassee, won a battle for the number two spot in preseason over Calvin Booker.