Late goals from Juan Zuniga and Troy Deeney gave the Hornets a 3-1 victory in September 2016, condemning the Red Devils to their third defeat in a week.
That period remains the lowest ebb of Mourinho’s Old Trafford reign and the United boss is aware just how hard it will be to exact revenge tonight.
Watford are eighth in the Premier League after their impressive 3-0 win at Newcastle, with manager Marco Silva’s fine work leading to admiring glances from rudderless Everton.
“Against Watford I was very upset with Watford’s first goal after some guy sent (Anthony) Martial to the hospital and the referee let it go,” Mourinho said.
“That is the reason why I was very, very upset at Watford. I think Watford made also progress. They have an unusual squad for a mid-table team.
“They have a squad full of very good players who are not normal to be in these mid-table teams. They have a good coach. They have improved a lot from last season too.”