Big dropoff from Manuel to Lewis?

Buffalo Bills quarterback E.J. Manuel still does not have an official timetable on the knee injury he suffered in last Thursday's game against the Cleveland Browns. Fellow rookie Jeff Tuel finished the game after Manuel went out but did not play particularly well in his first NFL action. The Bills brought up Thad Lewis from the practice squad earlier this week and now he'll make his second career start this Sunday against the Cincinnati Bengals. How big is the dropoff from Manuel to Lewis?