The Red Devils finished seventh by the end of the 2013-14 campaign and their new season under manager Louis van Gaal got off to a losing start in the form of a 2-1 defeat to Swansea City at Old Trafford on Saturday.
"Obviously people have been waiting for an opportunity to have a go at United for years and this is the perfect time," Fletcher told the club's official website. "It's a long season and let's see if they're saying that towards the end of it.
"I believe they won't be and that we're going to have a very good season. All the players believe that too. Losing the first game is disappointing of course, but comments like that gives us greater determination, without a shadow of a doubt."
The North-West outfit have been linked with numerous players, including Arturo Vidal, Angel di Maria, Daley Blind and Marcos Rojo, but are yet to bring in any more targets with two weeks left to go in the transfer window.