The midfielder is rested because of a hamstring problem, while the striker has been left out due to a shoulder injury sustained in Wednesday's Capital One Cup semi-final second leg win over West Ham on Tuesday.
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"Silva is not 100 per cent so will be rested and Negredo has a shoulder problem. We will see how Alvaro is over the weekend," Pellegrini told reporters.
"Silva has a small problem with his hamstring - nothing too serious - but he won’t play tomorrow as a precaution.
"The initial opinion on Negredo's shoulder injury is that it is not a serious problem. The rest of the squad is available."
Watford sit a disappointing 15th in the Championship - 10 points adrift of the play-offs - but Pellegrini is expecting a determined effort from Giuseppe Sannino's side.
He added: "Watford may not be having a great season but they will be motivated when they play us because this is the FA Cup."