Mazzarri says it was critical after their win at West Ham the previous week.
He told Radio Anch'io lo Sport: "We are very pleased with the success against Manchester United because it gives continuity to the performances that we have made so far. It also increases the self-esteem of our players.
"Jose Mourinho? I do not look at these things anymore. I'm happy because we won against a super team, beyond who is the coach. It's been 30 years since Watford beat United, but we coaches look at other things, the numbers and statistics we let the fans take care of.
"Watford's goal? To stay up and lay the foundations for growth."