The Hammers often save their best for the visit of the Red Devils, while the locals ensure the tasty action on the field is matched by vociferous support in the stands.
Despite a hostile reception in store for Fletcher and co, the Scotland international believes the champions will have little problem dealing with whatever Gianfranco Zola's side throw at them.
"We are the Premier League champions, European champions and world champions. West Ham want to turn us over and their fans will be right up for it," explained Fletcher.
"The atmosphere down there is hostile. Sometimes you watch a game on TV and it seems fine but for us it is different somehow.
"When you are successful people try to knock you down. Long may it continue is all I can say."
Meet the challengeFletcher knows the home support will be in United's faces, but the midfielder believes the atmosphere will bring the best out of Sir Alex Ferguson's troops.
He continued: "You must meet the challenge head on. If you go to take a corner and you are getting stick you just have to laugh it off and respond in the best way possible by winning the match.
"You need to channel it into a determination to prove them wrong. That is what we tend to do."
Catch all the action from Upton Park as West Ham entertain Manchester United on Sunday live on Sky Sports 1 and HD 1.