Gerard Deulofeu has been sidelined by a virus.
Watford's recent troubles deepened this week after four players were struck down with a stomach virus.
The Hornets, who host Crystal Palace in the Premier League on Saturday, are looking over their shoulder after losing four of their last five matches.
Gerard Deulofeu, the forward on loan from Barcelona, and defender Sebastian Prodl have both been absent from training due to the illness sweeping the camp.
Manager Javi Gracia did not wish to reveal the names of the other two players who were affected, but hopes all four will be fit for the visit of Roy Hodgson's side.
"Gerard was sick this week and there were other players who have been ill, too," Gracia said. "Prodl has also been ill all week.
"It is a stomach virus. They are recovered, available for the match and I am sure they are in the best condition."
The raft of illnesses will come as a dent to Gracia's preparations as his out-of-form Watford side attempt to halt their run of bad results.
Gracia enjoyed an impressive start to his tenure at Vicarage Road, but his side have taken just one point from the last 15 available.
The Hornets are nine points above safety, having played one game more than Southampton, but will be overtaken by Palace if their opponents win at Vicarage Road on Saturday.
Tom Ince scored in the 90th minute as Huddersfield secured a 1-0 victory against Watford last week, while Burnley came from behind to beat Gracia's team in the previous fixture. Jermain Defoe struck at the death to rescue a point for Bournemouth at Vicarage Road last month.
"In the last five games football has been cruel to us," Gracia added. "We have had many positives from those matches.
"We made mistakes but we have had good performances and in this moment we are confident we will be able to get a better result from the next game and we are working with that mentality.
"We need to win the next game because my players need that reward."
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Source: espn.co.uk