Harper was replaced by Tim Krul at half-time in last weekend's 3-0 win over Swansea after his back stiffened up, but will be fit.
Striker Andy Carroll has returned to training after recovering from the virus which sidelined him on Saturday, while midfielder
Nicky Butt is also available after a troublesome calf injury.
Shola Ameobi could be involved for the first time since the end of August after a second week of training following a long-term foot problem.
Only Joey Barton remains on the sidelines with his own foot injury, leaving manager Chris Hughton with a series of difficult decisions.
Watford skipper Jay DeMerit is closing in on a return ahead of the trip.
The American centre-half has been sidelined since August after needing eye surgery when a piece of grit got stuck under a contact lens.
DeMerit has resumed full training this week but although he is well ahead of schedule, Watford will be careful not to take any unnecessary risks with the player who turned 30 on Friday.
Teenager Ross Jenkins is still sidelined after a back operation but fellow midfielder Don Cowie returns to the squad after missing Saturday's 3-0 defeat at Crystal Palace with a calf injury.
Heidar Helguson and Danny Graham continue in attack despite drawing a blank at Palace.