United's match against the Hornets comes three days after facing Arsenal in the league and is the first of six games in 19 days for Louis van Gaal's side in March.
They include two crunch Europa League ties against Liverpool, an FA Cup sixth round versus West Ham as well as a derby at Manchester City, which could prove pivotal in the race for the Barclays Premier League top four.
The Red Devils are also crippled by injuries, with 14 first-team squad players currently out or struggling for fitness and Watford, who sit 10th after their goalless draw at home to Bournemouth on Saturday, could be well-placed to take advantage
"Sometimes when we plan for the match it's like we always need to be inside of the manager's head on the other team," Flores said.
"I don't know exactly how Van Gaal is thinking about the next match or what his focus will be.
"They have a lot of targets in this moment and play a lot of matches. For us it is an opportunity to have points.
"We know we are very competitive and we can go every single step to win. Now we have 37 points and we are happy."
Watford have enjoyed an excellent season so far and victory over United would see them reach the supposed safety-mark of 40 points.
Etienne Capoue, however, may be rested for the clash after Flores substituted the former Spurs midfielder against Bournemouth and suggested he might be suffering from fatigue.
"Capoue is a very important player for us," Flores said.
"He has played a lot of minutes this season. I felt he might be a bit tired. I felt he was not clear in the mind, he was not normal but it is possible when a player plays a lot he is a bit tired.
"On Sunday we will talk with him but I am not worried."
Source: PA