Newcastle United 0 Watford 3: Fourth successive defeat leaves club close to relegation zone
Watford ran out convincing 3-0 winners at St James's Park thanks to strikes from Will Hughes and Andre Gray and an own goal from DeAndre Yedlin.
The defeat leaves the club 13th in the Premier League and four points above the relegation zone.
Newcastle, without the suspended Isaac Hayden and injured trio Jamaal Lascelles, Christian Atsu and Paul Dummett, were second-best to Marco Silva's side in every department.
United lacked conviction in front of goal and creativity in midfield.
They were also poor defensively, and the ease with which Watford scored their goals will be a concern for Benitez ahead of Tuesday night's game against West Bromwich Albion at The Hawthorns.
Benitez recalled Mohamed Diame to his starting XI in place of Hayden. Fit-again Mikel Merino had to be content with a place on the bench alongside Aleksandar Mitrovic.
The first chance of the game fell to Joselu, who miskicked after Jacob Murphy broke down the left and crossed to him. Matt Ritchie also stuck the side netting in the 12th minute.
Watford took the lead in the 19th minute. Marvin Zeegelaar broke down the left and his cross was swept home by Hughes.
Joselu had a shot blocked by Adrian Mariappa and another effort saved by Heurelho Gomes.
Zeegelaar capped a superb first-half performance with a low cross that led to Yedlin's own goal.
There were some boos at the half-time whistle.
United never looked like coming back after going two goals down, and Gray netted a third for the visitors in the 62nd minute. The striker, a summer target for Newcastle, beat Elliot from close range after Richarlison crossed from the left.
Manager Benitez sent on Merino, Mitrovic and Ayoze Perez in the second half, but the damage had been done.
And there were more boos when referee Christopher Kavanagh called time on the game.
NEWCASTLE UNITED: Newcastle team: Elliot; Yedlin, Clark, Lejeune, Manquillo; Ritchie, Diame (Merino, 69), Shelvey, Murphy (Perez, 77); Gayle, Joselu (Mitrovic, 69). Subs not used: Darlow, Gamez, Mbemba, Aarons.
WATFORD: Gomes; Mariappa, Kabasele, Britos (Prodl, 52); Femenia, Cleverley, Doucoure, Zeegelaar; Hughes (Carrillo, 84), Richarlison (Pereyra, 79); Gray. Subs not used: Karnezis, Janmaat, Watson, Capoue.
Goals: Hughes 19, Yedlin (og) 45, Gray 62
Bookings: Shelvey 15, Zeegelaar 24
Referee: Christopher Kavanagh (Lancashire)