Watford squad decimated by stomach virus
Gerard Deulofeu, on loan from Barcelona, and Sebastian Prodl are among the players to have been struck down, with Gracia reluctant to name the other two players suffering with the problem.
Nevertheless, the former Malaga boss is hopeful that all four will be fit for Saturday's game.
"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."
Watford face Palace having lost four of their last five Premier League matches, but the Spaniard is hopeful his side can turn things around.
In the last five games football has been cruel to us," Gracia admitted. "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."