With just nine points separating United in fifth, from West Bromwich Albion in thirteenth, the race for the final Europa League place could be as tight as ever this season.
Though Flores' side have lost just once in their previous five league games - against title favourites Tottenham Hotspur - they have won only two of their last ten fixtures.
A reason for Watford’s inconsistency may be that of the form of top scorer Odion Ighalo, who has just one goal in 2016. The Watford faithful will be hoping the Nigerian forward can find his form in time for the trip to Old Trafford.
Ex-Atletico Madrid boss Flores almost has a fully fit squad at his disposal with just Jose Jurado (calf) and Joel Ekstrand (knee) unavailable. Ettienne Capoue, who has been indispensable since his arrival from Spurs, may be rested after suffering from fatigue in the goalless draw with Bournemouth last time out.
Manchester United boss Louis Van Gaal will certainly be envious of his Watford counterpart. The Red Devils have been stunned by a number of injuries, with up to 17 players currently spending time on the treatment table. Phil Jones, Luke Shaw, Cameron Borthwick-Jackson, Ashley Young, Marouane Fellaini, Bastian Schweinsteiger and Will Keane are all expected to miss out while captain Wayne Rooney remains in a leg brace. Chris Smalling, Anthony Martial and Matteo Darmian will be given late tests to prove their fitness.
Such injuries have forced Van Gaal to dip into the talent available from the United academy, with striker Marcus Rashford grabbing the headlines in the past week. The 18 year old scored twice on his debut against Midtjylland, before smashing another brace in the 3-2 triumph over Arsenal.
Speaking at his pre-match press conference, Flores recognised the threat the Rashford could cause, suggesting he "looks really good" while highlighting his youth. Flores also suggested that he loves how Rashford "moves to the space" and "how he moves inside the box".
"It's an opportunity to establish themselves"
The Spaniard also acknowledged the impact that the United youngsters have had, suggesting that United have "had a lot of injuries", resulting in "a very difficult period" but that the moment has now arrived when "the younger players" can "use the opportunity to establish themselves" in the first team squad.
Mindful of the effect it could have on his players, he added that "yes, they are younger", suggesting that the young players could "have a lot of passion" as they try to prove themselves. Flores then suggested that "maybe this is not the best XI they have in the squad", but also admitted that United are playing "really well".
Meanwhile, Van Gaal has urged the media not to build too much hype around Rashford, suggesting that they "leave him in peace" and not put too much pressure on the youngster. The Dutchman was quick to suggest that Rashford was getting "too much attention", and that "it's not always easy to deal with." He did, however, admit that Rashford's rise has been "remarkable" and that he could "understand" why he has been given such praise.
Watford have never won a league match at Old Trafford in eight previous attempts. They have, however, won in the League Cup, with Luther Blissett scoring both goals in 1978’s 2-1 win.
United have also won the previous ten encounters, the most recent victory coming in the form of a 2-1 win courtesy of a Troy Deeney own-goal in the dying embers of the corresponding fixture.
Watford have failed to score in five of their last seven Premier League games. They have been over-reliant on strike duo Deeney and Ighalo who have 22 of their teams 29 goals. Ighalo has 14 whilst skipper Deeney has eight.