Deeney underwent a knee operation on Thursday and is expected to be sidelined for several weeks.
Meanwhile summer signings Danny Welbeck and Ismaila Sarr could feature for Gracia, but Roberto Pereyra (thigh), and Abdoulaye Doucoure (ankle) are doubts.
The Hornets have endured a tough start to the season and are currently just one of four teams that sit on no points after their first two outings in the Premier League.
Watford were shocked by a 3-0 defeat at Vicarage Road on the opening day of the season as Brighton got their campaign off to a blistering start.
Gracia will have been more pleased with his team’s efforts as they battled to a 1-0 defeat at Everton last time out but the Spaniard will be determined to see his side score their first goal and earn their first points of the campaign.
As for West Ham, they are set to be boosted by the return of their own captain Mark Noble who has missed the opening two matches with a calf problem.
Sebastien Haller and Felipe Anderson will also be available for selection after they were ruled out of last weekend’s 1-1 draw at Brighton through injury.
Midfielder Jack Wilshere is likely to be in Manuel Pellegrini’s squad despite failing to participate in the second half against Brighton.
The former Arsenal midfielder has endured a host of injury problems since his move to the London Stadium from the Emirates and he will be hoping to have a clean bill of healthy through most of the current campaign.
Pellegrini will be looking for a similar performance from his side to their last trip to Vicarage Road, as they secured a 1-0 win at the home of the Hornets.
Noble bagged himself a brace that day, while Manuel Lanzini and former Hammer Marko Arnautovic also go on the scoresheet against Gracia’s side back in May.
Provisional Watford squad: Foster, Femenia, Dawson, Cathcart, Holebas, Hughes, Doucoure, Capoue, Sema, Gray, Pereyra, Deulofeu, Gomes, Janmaat, Cathcart, Cleverley, Okaka, Welbeck, Chalobah, Quina, Masina, Sarr.
Provisional West Ham squad: Fabianski, Fredericks, Balbuena, Diop, Masuaku,Rice, Fornals, Wilshere, Noble, Lanzini, Anderson, Haller, Anderson Roberto, Zabaleta, Ogbonna, Cresswell, Sanchez, Snodgrass, Yarmolenko, Antonio, Hernandez, Ajeti.