Many believe defeat could be too much for United to haul in table leaders Chelsea.
But Fletcher says: "Sunday is a massive game. Games against Chelsea always are.
"It is not going to make-or-break who wins the league by any means, although it is an important benchmark.
"Maybe psychologically it might have an effect. But Chelsea are a top-class side and we will respect them."
He also remarked: "We have had a couple of draws down there but we have still not managed to win in the league and we are desperate to do it.
"But we will go there with a game-plan and try to get a result."