Capoue is one of just two players to feature in every Premier League for the Hornets this season, along with Heurelho Gomes, but missed training on Wednesday and Thursday due to a fever and he will be assessed on Friday.
Right-back Janmaat (groin) and midfielder Nordin Amrabat (ankle) are back in contention - Amrabat for the first time since New Year's Day - but Christian Kabasele and Ben Watson are still not fit.
Roberto Pereyra and Mauro Zarate are long-term absentees and will miss the visit of the resurgent Eagles, who won their last two matches without conceding a goal to climb out of the relegation zone and breathe new life into their Premier League survival bid.
Boss Walter Mazzarri believes Palace's position in the table is false and he expects them to be a threat to Watford at the weekend.
"Palace are one of those teams that, when you look at their squad, nobody thought when we started they would have fought relegation," said Mazzarri at his pre-match press conference on Thursday.
"The game will be decided by individuals. In order to have a positive outcome my players have to come to the pitch with their legs and their heads in perfect condition.
"They are on a good run, are in good form and are in good condition. We will play them in their best moment as they come from two positive results. They've got an important fan base so it won't be easy.
"We go to the pitch knowing we cannot commit any mistakes. We have to go there and give everything in order to come out with a positive outcome.
"From now, all games will be extremely difficult and different from the first half of the season. They will all be battles, they will all be finals."
Provisional squad: Gomes, Pantilimon, Britos, Kaboul, Prodl, Mariappa, Cathcart, Zuniga, Janmaat, Holebas, Cleverley, Behrami, Amrabat, Doucoure, Capoue, Deeney, Success, Niang, Okaka.