The Icelander is back at Vicarage Road on loan from Rangers and cannot face his parent club.
Boss Malky Mackay will check on the fitness of Arsenal loanee Henri Lansbury, who limped out of Saturday's 2-0 defeat at Newcastle with a hip injury.
The Hornets have been boosted by the return of club captain Jay DeMerit, who was on the bench at St James' Park.
The USA centre-half has been sidelined since August after needing surgery on the eye he scratched while removing a contact lens.
Teenage midfielder Ross Jenkins is therefore the only definite injury absentee following his recent back operation.