Fletcher will never forget his spell at Manchester United where he progressed through the youth ranks and got the opportunity to play with some of the best young talent in England.
The 21-year-old enjoyed a successful loan spell with Barnsley last season before making a big move to West Ham United during the summer.
Fletcher has already showed his quality for the Hammers in the Premier League and says it will be an emotional day when he lines up against his former team-mates for the first time.
Fletcher said: “I am looking forward to going back to Old Trafford for the first time and it will be great to meet up with a number of old friends.
“I spent the majority of my youth career at Manchester United and learned a great deal during my time there so it will be good go back and show what I can do.
“This is probably the first time I have played against Marcus in a competitive game. I am looking forward to seeing him and other players like Jesse Lingard.
“I still speak to Marcus and Jesse and see them when I am back in Manchester. They have progressed really well and have broken into the England team.
“I have to praise them for the way they have made it through the Academy and are now playing for the national side. We will have to put our friendship to one side on Sunday.
“They are the players I grew up and have played alongside. It will be a huge occasion for my family and friends and for the Manchester United fans to see how far I have come. Hopefully I can put on a good performance.
“All my focus will be on West Ham and helping us to come away with a good result on Sunday. I am hungry to get more chances and help us move clear of the bottom three.”
Fletcher's last appearance for United at Old Trafford was a game he will never forget as he played a major role in a 4-0 win over arch rivals Manchester City.
The talented forward would love to get on the scoresheet once again and make amends for a disappointing week after his side suffered a dramatic late defeat at White Hart Lane last weekend.
Fletcher added: “That was a good memory for the U21s and it was the season before I joined Barnsley. It was not a bad way for me to play my last game there!
“Last week was disappointing because there were only a few minutes left on the clock and we were unable to hold onto the victory.
“It now makes it even more important for us to get something out of the game at Old Trafford.”