Watford fought off a Manchester United comeback before two late goals to secure their first win against the Red Devils in three decades.
The home side started off brightly and completely bossed the opening exchanges before strikers Odion Ighalo and Troy Deeney both missed good chances with less than 15 minutes played at Vicarage Road.
David De Gea pulled off a fine save to deny Deeney with a strong header before United’s first effort through Zlatan Ibrahimovic was also well saved.
With the 30 minute mark approaching, Paul Pogba tested Heurelho Gomes from range but the crossbar denied the Frenchman the opener.
Four minutes later, Watford took the lead despite Anthony Martial being fouled in the build up to Etienne Capoue smashed his fourth goal of the season as Watford went into the break 1-0 ahead.
Ibrahimovic was denied twice before Marcus Rashford pulled his side level as the Red Devils began to control the game and chased the win.
With the visitors controlling the tempo and looking the more likely of the two to find a winner, Watford introduced Juan Zuniga off the bench and within a minute, he stole the headlines and the win as he slotted past De Gea, who was left helpless by some lacklustre defending in front of him.
With United chasing the game but unable to create any chances or control possession as the clock wound down, Rooney and substitute Memphis were both booked for dissent under the league’s new rules.
Hornets captain Troy Deeney capped off the win with an injury time penalty which he blasted past De Gea following a clumsy challenge by Fellaini.
The loss, Mourinho’s United’s third in the week, means the side from the red half of Manchester are now six points behind their rivals and league leaders Manchester City.
Mourinho’s side are now sixth behind Liverpool with Spurs still to play on Sunday.