Fletcher, 21, opted to leave Old Trafford during the summer having failed to make a single appearance for the senior team. Last January, he was given the chance to make a name for himself on loan at Barnsley, seizing that chance and helping fire the Tykes to promotion to the Championship.
But while he was away, a striker shortage allowed a player unknown outside Manchester by the name of Marcus Rashford to rise to prominence.
Having made nine appearances for Slaven Bilic's side this season, Fletcher already believes his decision to leave Manchester has been vindicated.
Speaking to Sky Sports, the striker also revealed the extent of United's attempts to keep him at the club, having discusses his options with the club's head of academy Nicky Butt and executive vice-chairman Ed Woodward.
"The decision to leave was probably made more or less before Jose had come in," Fletcher said. "I had obviously spoken with Nicky Butt, who was the head of the academy at the time, Warren Joyce and people who were above in the board, John Alexander and Ed Woodward. I just thought the decision was right for me and for my family; it was time for me to go and make a career for myself.
"Manchester United offered me a new contract, but in terms of what I wanted, it was not right. It wasn't about anything financial, it was just about my career and my progress. What they were offering and their ambitions for me did not match mine."
Fletcher is likely to be part of the West Ham squad travelling up to Old Trafford for their Premier League clash with Mourinho's side on Sunday (27 November).