Jose Mourinho praised the Hornets' for what was a brave performance by the home side. It means that Watford drop down to fourth in the English Premier League.
Quick double seals the victory for United
For half an hour, the match at Vicarage Road was very open with both teams creating many chances - including an offside Alexis Sanchez goal within the first few minutes. Hornets captain Troy Deeney was denied by a great save from United goalkeeper David de Gea, whilst Ben Foster was tested on numerous occasions by the United attack.
However, the Hornets' defence was undone in the 35th minute, when Romelu Lukaku tapped away a goalmouth scramble and handed the away side the lead.
3 minutes later, after a great save from Foster to deny Paul Pogba, the Red Devils doubled their lead - again from a set piece. Marouane Fellaini rose highest, and Chris Smalling finished excellently, giving Foster no chance.
There could have been a third before the break if it wasn't for Foster's acrobatics once again to deny Pogba from finishing from close range. United went into the break cruising.
Watford came out from the half-time break stronger, where being in front of Vicarage Road's Rookery Stand clearly gave the players a boost. However, it wasn't until the 65th minute before Andre Gray blasted home from the centre of the box into the top right corner, giving Watford fans hope.
Hope proved to be the only thing Watford fans had, however, as chance after chance went by with nothing happening. Chances were given to the away side as well, but if there was going to be a fourth goal to the game, it would have gone to the home team.
And there so nearly was an equaliser for Watford in the last few seconds of injury time. Immediately after Nemanja Matic was sent off for a second yellow card, De Gea proved he is still very much one of the best goalkeepers in the world, if not the best, as he spectacularly saved Christian Kabasele's header which was destined to go into the top-right corner.
Mike Dean blew his whistle for full-time after another Watford chance was saved by De Gea. The Hornets' should be proud of their attempted comeback, whilst Manchester United should be happy that they managed to narrowly hold on for a 2-1 victory.
Fellaini leads a strong midfield forward
Fellaini has been subject to much criticism in the past, but today, he showed how good he can be, alongside his other midfield counterparts.
From start to finish, the Belgian midfielder led the United midfield line, and from every set-piece, he was a threat - with nearly every ball pumped into the box usually finding the former-Everton man, who helped his side create many chances inside the box throughout. His work effort paid off as he assisted Smalling's volley to give United the two-goal cushion.
But it was not just his work effort going forward that made the difference. He tracked back whenever he could, and was making clinical passes throughout. Fellaini has never been rated very highly, but when he has a good game, he can have a very good one. And today proves that factor.
On what was his second start for the Reds this season, Fellaini will make it hard for anyone to replace him when he and his squad host newly-promoted Wolves to Old Trafford on Saturday - or even in their Champions League tie against Swiss side Young Boys in midweek.
Pogba and Sanchez were also key to Mourinho's gameplan this week, with both playing some key passes and creating many a chance. As stated, Pogba was expertly denied twice by Foster, whilst Sanchez made some key runs (especially in the first half) down the left-hand side.
Watford's perfect start ends
Prior to today's kick-offs, Watford sat 3rd in the Premier League, and one of only three teams to have a perfect start to the season after four games played; Liverpool and Chelsea being the other two.
They had impressed after they turned their match against Tottenham around - beating them 2-1 thanks to goals from Deeney and Craig Cathcart.
Many fans were hopeful that the Yellows would make it five wins in a row for what would have been the first time in the club's Premier League history; and would have also put them on to 15 points. It was not to be that way for Watford, however, and the away side held them off for the final twenty-five minutes.
On occasion, the home side was sloppy and were giving away possession far too often. However, the courage that they showed in the second half was noticed by every fan in the crowd, as they stayed behind after the match to applaud their team's performance. It was certainly not the best game played by Watford today, but to go attacking against a side as strong as United is something that not many sides would have the courage to do.
The United number one proved to be the difference between Watford claiming a point or nothing at all. All the second half pressure so nearly paid off, but yet another stunning save from the Spaniard handed his team the three points.
Most Hornets' fans left Vicarage Road with their heads held high tonight. The players' should do the same as well. A test at Craven Cottage next Saturday awaits in the early Kick-off, and the squad should feel happy with the desire they showed in the second half today.
Things must be worked on during training, though. Both goals from the away side were from corners, which means that 3 of the 5 goals conceded by Watford so far this season have come from them. Javi Gracia knows that this is a key area that Watford need to work on, and he must do so this week.
Both managers will be happy with their team's performance after today's late kick-off. Mourinho will be hoping for an ideal start to Manchester United's Champions League campaign on Wednesday night in Switzerland, before hosting Wolves - where he will try and make his team win a third game on the bounce in the top flight.
Gracia will be hoping for a performance much like what the second half was today when Watford travel to South London to face off against Fulham; where they will be hoping to regain the momentum that the club has started off with this season