The Hornets are big underdogs for the third round tie at Old Trafford given they are sixth in the Championship and United are second in the Premier League.
Munoz, however, has told his player to rise to the challenge and see if they can pull off an upset by winning on Saturday night.
“The most important thing is we go there and fight like animals and enjoy the situation,” said Munoz.
“I know it will be difficult, because Manchester United is a perfect team, but we need to give our best and try very hard, because the most important thing is that our fans enjoy watching our team.
“We go there with maximum ambition, we will work very hard and it is important to see we are growing game by game. This is our objective and it’s important we continue with the same attitude, work and spirit.
“Everyone knows it’s a special competition, I’m very happy to be here and try to get us one step further because it’s very important.
“We know it’s difficult, but we will give our maximum in the next game and we will try to get to the next step of the competition.”