Still under the control of Ole Gunnar Solskjaer, who had clung onto his position over the international break, the visitors were, despite their terrible form, the overhelming favourites for the tie, with many viewing Watford as easy meat for a Man Utd side much in need of a win.
Yet, when Jonathan Moss blew his whistle for kick off at 3pm on Saturday, it was clear that the Watford players hadn't received that memo, putting on an utterly dominant display in front of the Vicarage Road faithful.
After missing a number of chances, including a penalty that was re-taken and missed again by Ismaila Sarr, Watford took a deserved lead through Joshua King in the 28th minute, before Sarr made up for his earlier penalty miss by firing the Hornets two ahead on the brink of half-time.
Half-time substitute, Donny van de Beek pulled a goal back for United in the 50th minute, but their momentum was halted when Harry Maguire was shown a second yellow card and sent off for a foul on Tom Cleverley.
Continuing to dominate and create chances, Watford made sure their performance got the score line it deserved, sealing three points with goals late on from Joao Pedro (90+2') and Emmanuel Dennis (90+4').
Imran Louza and Midfield balance
In the summer, Imran Louza was Watford's biggest outlay on any player, costing a reported nine million pounds when he joined from Nantes, yet his opportunities to make an impact have been very much limited.
Perhaps this was due to his poor performance in the second game of the season away at Brighton, where he was given an opportunity from the start, but at times, looked lost and as if the game was passing him by.
He was hooked at half-time and hadn't featured in a single minute of Premier League football since.
In the build up to Saturday's game, Claudio Ranieri acknowledged Louza's form for Morocco during the international break, but his comments suggested Louza was far from contention for a place in the starting line up.
“For me, Louza’s best position is as a playmaker in front of the defensive line. I saw him further up for Morocco, which was a good solution for them. I think in two, three months he could give us this solution." Ranieri said in Thursday's pre-match press conference.
He was superb.
An assured display at the base of Watford's midfield, a far cry from his performance at Brighton, finally showed why Watford decided to spend their money on him during the summer.
Not only that, but in occupying the deeper role in front of the defence, Louza freed up Moussa Sissoko, who has previously occupied the deeper role, and Tom Cleverley, who was brilliant throughout, to press higher up the pitch, something Watford have largely neglected this season.
This meant Watford were able to get at Manchester United and pin them back, particularly during the first half, when United, at times, genuinely struggled to get the ball out of their own half.
Watford's pressing, carried out by Cleverley and Sissoko, and facilitated by Louza at the base of midfield, led to chance after chance, and was a crucial part of Saturday's victory.
On Saturday, Louza felt like the missing jigsaw piece in a Watford midfield that has looked unbalanced at times this season. It will be fascinating to see if Claudio Ranieri sticks with him for the trip to Leicester next weekend. It would be hard to drop any one of the midfield three after that performance.
No clean sheet but defensive resilience
After Donny van de Beek's goal on Saturday, Watford have now gone 12 games in the Premier League without a clean sheet this season.
Stretching back to the 2019/20 season when they were relegated, it's 22 consecutive Premier League games without one for the Hornets.
Clearly, this is an unwanted run and something that will have to change if Watford are going to stay up this season.
The introduction of the experienced Nicolas Nkoulou against Arsenal seems to have helped things and acted as a calming influence at the back, and although it is very early days, his first two performances in a Watford shirt have been promising.
On Saturday, when Manchester United got one back, many may have expected them to equalise given Watford's defensive record. Yet, Watford's defence were resolute and ably supported by the midfield.
There are some tough games coming up, so it may be that Watford's wait for a clean sheet goes on a little longer, but as the weeks go on, there are signs of increasing defensive solidity under Claudio Ranieri and it will surely only be a matter of time before that first clean sheet comes.
Much needed victory amongst a difficult fixture run
Three points are always welcome, especially when you're towards the bottom of the Premier League table, but Saturday's victory came at a welcomed time for Watford, who find themselves in the middle of a particularly difficult fixture run.
It's become a bit of a cliché that there are no easy games in the Premier League, but Watford's current run is particularly tough.
Having played played Arsenal and Manchester United consecutively, Watford's next three opponents come in the form of the reigning FA Cup champions Leicester City, reigning European champions Chelsea, and reigning Premier League champions Manchester City.
During such a run, any points are a bonus, which is why Saturday's victory is all the more satisfying, and came at a great time for Watford, who were slipping closer and closer to being in the bottom three for the first time this season.
After Saturday's victory, Watford are four points clear of Burnley in 18th place, and if Watford can remain outside of the relegation zone whilst on this tough fixture run, it will be a big boost to their survival chances.