United were 3-0 up before half-time at a freezing Vicarage Road thanks to a double from Ashley Young and a goal from Anthony Martial.
But it almost proved to be a game of two halves as the Hornets struck twice after the restart through substitute, Troy Deeney’s penalty and Abdoulaye Doucoure before United’s Jesse Lingard finally put the points in the bag four minutes from time.
United’s victory, on the back of two successive away defeats, leaves them five points behind Premier League leaders, Manchester City, who are at home to Southampton later today.
Though the Portuguese boss said his side had “lost stability” after midfielder Nemanja Matic went off early in the second half with a “muscular” injury, he was still left frustrated with the second-half slip.
“The second half should have been five or 6-0, it was easier to score than the first,” Mou said.
“It was an easy match to win comfortably. At 3-2 the game is in the balance then Jesse Lingard had the initiative to kill the game.
“Both him and Ashley Young scored great goals. We missed 4-0 with some actions and Romelu Lukaku had an open goal, Paul Pogba had a free header too…We should be smoking cigars but then were in trouble.”
“We lost stability when Nemanja Matic had to come out but the important thing is the three points.
“I saw happiness from the team and played with a free mind.
“Last season I learned what Ashley Young can be, he gives stability in his performances and this season he is playing more regularly.
“He has the spirit and experience we need, today he also had the talent to score two beautiful goals.”
Meanwhile, Tottenham’s fading title hopes were finally extinguished by a 2-1 defeat at Leicester thanks to goals from Jamie Vardy and Riyad Mahrez.
Vardy grabbed a 100th career league goal when he volleyed a dinked pass from Marc Albrighton, before Mahrez drilled an unstoppable shot into the top corner in first-half stoppage time.
Harry Kane netted his 16th club goal of the season in the 79th minute, but fifth-placed Spurs couldn’t muster an equaliser and remain 13 points behind leaders, City.