The Newcastle manager will still be without Jonjo Shelvey, Paul Dummett and Mohamed Diamé, but was pleased to have Fernández and Kenedy returning from the sidelines.
Both players were unavailable for the Magpies' Premier League win over Cardiff City due to illness but could be selected for the cup tie against the Hornets at St. James' Park.
"Shelvey, Dummett and Diamé are getting better," Benítez said. "Ki (Sung-yueng) was a surprise for us because he was coming back from the Asian Cup with an injury.
"He could be out for a couple of weeks if everything is fine. Fernández and Kenedy are fine and have been training this week."
Prior to United's convincing win over the Bluebirds, Benítez's side completed a 4-2 cup victory over Blackburn - taking their goalscoring tally up to seven in two matches.
And the United boss insisted continuing their form in front of goal could be crucial crucial if United are to end a 13-year wait in reaching the fifth round stage of the prestigious competition, with the Magpies last reaching the quarter-finals in 2006.
Benítez added: "This group of players are working hard and we can get some results against teams - it has to be like that. Scoring seven goals in these (last two) games is massive for the confidence of the players. You can score goals, control games and you can play well so, hopefully, we can keep this momentum.
"Every game is different. We know Watford are a good team but if we do well and the fans are behind the team, we have a chance to go through (to the fifth round).
"We have to use the squad. We have another important game in a few days (against Manchester City). Both games are important, and we have to try to manage a strong team for both games to go through in the FA Cup.
"If we can win this game, you never know what can happen. I am quite positive and optimistic. It is clear that teams can select a strong team trying to go through because you never know what will happen in the draw so, for us, we will try to manage and, for Watford, I expect them to come with a strong team."
