United have endured a miserable start to the season, losing two of their first four matches and slipping well off the searing pace set by Liverpool, Chelsea and Watford themselves.
But two goals in three first-half minutes from in-form Romelu Lukaku and Chris Smalling gave them valuable breathing space at half-time and ultimately paved the way to a victory that ended Watford’s perfect record.
Rocked by United’s strong finish to the first half, Watford pulled a goal back midway through the second period through Andre Gray but failed to find an equaliser despite a strong finish.
Mourinho opted for power rather than guile at Vicarage Road with a physically imposing midfield trio of Paul Pogba, Marouane Fellaini and Nemanja Matic.
Former Watford player Ashley Young came in at left-back to replace Luke Shaw, who suffered a head injury while on England duty last week.
The first half an hour was even as the two sides tested each other out –- United probed but the home side looked dangerous on the break, confidence coursing through the players on the back of four straight wins.
Watford went close to breaking the deadlock when Troy Deeney forced David de Gea to tip over just after the quarter-hour mark.
Midway through the half Alexis Sanchez had the ball in the net but it was ruled out for offside, and minutes later the Chilean forced a fine save from Watford goalkeeper Ben Foster following a jinking run.
United, backed by their vocal travelling fans, who regularly chanted the name of Mourinho, were establishing a foothold in the game and made their pressure count in the 35th minute.
Watford failed to clear a free-kick and when the ball came back in, Lukaku was on hand to bundle the ball in off his midriff to score his seventh goal as many games for club and country this season.
Foster then kept out a powerful Pogba drive but United were two up in the 38th minute when Smalling controlled a Fellaini header following a corner, and smashed the ball home.
Buoyant United then nearly grabbed a third but Foster saved well from Pogba as Watford kept the score at 2-0 at the break.
But Javi Gracia’s side came right back into the game in the 65th minute when Abdoulaye Doucoure picked out Gray, who smashed home, giving De Gea no chance.
That goal infused the team and the fans with renewed belief and United suddenly looked nervy.
Doucoure saw a late left-foot shot from outside the box saved at the near post as time ran out.
Matic was sent off for a second yellow card but Watford did not have time to make their extra man count, with De Gea pulling off an outstanding save to tip away a Christian Kabasele header in the dying seconds.
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