The Irish centre-half returned on the bench at Coventry last Saturday, but suffered a recurrence of his shoulder injury in training this week and could need another operation.
Claude Davis and Jose Fonte will therefore continue their partnership at the back.
Young winger Sean Scannell missed out last weekend after sustaining a hamstring injury playing for the Republic of Ireland Under-21s and remains a doubt.
Striker Calvin Andrew will be on the bench again after a substitute appearance against the Sky Blues - his first competitive action since sustaining a cruciate injury in May.
Left-back Lee Hills starts a three-match ban after being sent off while on loan at Oldham.
Ross Jenkins and Jay DeMerit remain Watford's only injury absentees for the trip.
Teenage midfielder Jenkins is continuing his recovery from a back injury and defender DeMerit remains sidelined after his recent eye surgery.
Boss Malky Mackay says the duo are "weeks rather than days away", so he will probably stick with the team which thumped Scunthorpe 3-0 last Saturday.
Heidar Helguson keeps his place in attack after scoring twice against the Iron to take his tally to five goals in four appearances since rejoining the Hornets on loan from QPR.
Back-up goalkeeper Richard Lee will be on the bench as cover for Scott Loach after recovering from the knee injury which sidelined him for five games.
Sporting Life