Midfielder Gape suffered a thigh strain against Forest Green on New Year’s Day and had to be substituted early in the second half.
Matt Bloomfield was taken off as a precaution in that game but will be available, while defender Adam El-Abd, who has not featured since mid-December, is likely to return from a calf problem.
Defenders Sido Jombati (knee), Anthony Stewart (calf) and Michael Harriman (broken leg) are out, along with forward Scott Kashket (pelvis).
Billy Bodin could make his Preston debut, but fellow new signing Louis Moult’s bow will likely have to wait.
Bodin was signed from Bristol Rovers on Thursday evening and is not cup-tied having missed their previous game in the competition.
Moult, recruited from Motherwell, has also completed his switch to North End but revealed he is carrying a slight injury that will prevent him from training until next week.
Eoin Doyle is back from a productive loan spell with Oldham yet a bout of illness means he will miss out along with Tommy Spurr (knee), Josh Earl (knee) and Sean Maguire (hamstring), while Darnell Fisher (hip) remains a doubt.
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.