Shelvey's last start for the Magpies came against the Hornets back in November, as the midfielder was forced off in United's 1-0 victory at St. James' Park.
And Benítez believes the central midfielder's return from the sidelines has come at a significant time follwoing Ki Sung-yeung's departure for international duty with South Korea, who compete in the Asian Cup next month.
"(Ciaran) Clark and (Karl) Darlow are both still unavailable," Benítez said. "However, the rest of the team is available, including Jonjo Shelvey. We have bodies but when you lose a player like Ki and Jonjo was injured, you are already losing two players who both have the technical ability to manage the game so I think it's very important to have everybody fit."
With Watford currently in tenth place at the halfway stage of the Premier League, Benítez was keen to praise the work of Hornets boss and fellow Spaniard Javi Gracia.
To end their 2018 campaign in a positive manner, United will be looking to improve their record against teams situated in the top half of the table, with the Magpies picking up four points out of a possible 30.
"I think we are talking about a good team and some people underestimate the great job that Javi Gracia is doing," Benítez added. "They are playing well and can compete against anyone. They can keep the ball, defend and are well organised.
"They have ability, strength and physicality so I think it's a good team who have a good manager who is doing a good job.
"For us, we need to approach each game like a final. We cannot see too much what is going around or what is happening in terms of the table.
"We just need to concentrate on the next game and getting the next three points. We cannot think too much about what we have to do in the next two or three matches - it has to be the next one.
"It is important for everyone to end 2018 on a high note. Getting three points would be massive as it would put us in a much better position and also in terms of confidence for the rest of the season but it is not just to finish the year well but also to start 2019 positively."
- Rafa Benítez Highlights before Cardiff
- Rafa Benítez Highlights before Blackburn
- Rafa Benítez Highlights before Chelsea
- Benítez: "It's an opportunity to do well"
- Rafa Benítez Highlights before Watford
- Benítez: "It's important to end 2018 in a high note"
- Rafa Benítez: Highlights before Fulham
- Benítez: "We are in a good position"
- Rafa Benítez in The Sunday Times
- Benítez: "Every game is really important"