Some pundits have branded the current United team the worst in 20 years after their 2-1 defeat by Swansea City on the opening day of the season.
But Fletcher thinks United's critics have been desperate to take pot shots at the club as a consequence of their recent success and the team's vice-captain thinks they will be eating their words at the end of May.
"People have been waiting for an opportunity to have a go at United for years and this is the perfect time," said Fletcher.
"It's a long season and I don't think they will be saying those things at the end of it.
"I believe we're going to have a very good season. All the players believe that too.
"Comments like that give us a greater determination, without a shadow of a doubt."
Having been part of the United team that finished seventh last year, Fletcher knows how it feels to be on the receiving end of some scathing criticism.
The midfielder also remembers when Alan Hansen told Sir Alex Ferguson "you can't win anything with kids" in 1995.
Ferguson won the league that season and scooped a further 10 titles after building a team around a group of youth players from the famed 'Class of '92'.
Fletcher thinks Van Gaal's squad has a similar look to it as the one that contained the likes of David Beckham, Paul Scholes and Ryan Giggs just over a decade ago.
"To be honest it felt like that in the dressing room today," said Fletcher, who played alongside debutants Tyler Blackett and Jesse Lingard on Saturday.
"It did have a feel of that in terms of people looking at the squad and maybe not seeing household names, but these players are good enough and the manager believes in them.
"All you need is a platform to go and perform and I'm sure they'll definitely go and prove people wrong.
"This whole club is about bouncing back from adversity."
Van Gaal has lost experienced heads like Giggs, Rio Ferdinand, Nemanja Vidic and Patrice Evra from his playing squad this summer.
But Fletcher thinks the players that remain are strong enough personalities to fill the void.
He said: "People talk about the players that have left but I see it in the eyes of the young players, they're ready for it and determined to show people how good they are and we're determined to show people how good the team are.
"You have to be a big character to play for this club."