The Scot returned to United's midfield for the UEFA Champions League draw with CSKA Moscow on Tuesday after a month's absence with an ankle injury, and was controversially booked for diving in the penalty area when he appeared to be fouled.
With the Red Devils trailing Premier League leaders Chelsea by two points, a win at Stamford Bridge would see them go top and Fletcher is relishing the challenge posed by Carlo Ancelotti's side.
"Sunday is a massive game, games against Chelsea always are," he said.
"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. It would be good to win.
"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."
PraiseFletcher has been praised for his performances so far this season and is now recognised as one of the first names on the team-sheet at Old Trafford, although the 25-year-old is not getting carried away with the plaudits.
"I take the compliments with a pinch of salt," he added.
"I enjoy these matches as much as anyone and I put pressure on myself to perform more than anyone does.
"The challenge of playing against the top-class players that Chelsea have is one I look forward to and I will be doing my best to win the match.
"But I am not going to make-or-break a game like this.
"It is nice to get back into the team (against Moscow) and get 90 minutes under my belt because I have not played for a while.
"I felt fine, which was the most important thing, although I was not 100% with my touch and awareness on the pitch. But the game will help and, if selected, I should feel the benefit on Sunday."