Rashford scored after coming on as a substitute against Bournemouth at the weekend, while Romelu Lukaku, who missed the game through injury, hasn't scored in his last nine appearances for United.
Fletcher, now of Stoke, said of United: "They should treat the game as they did in the second half last season. They were brave and tried to get up the pitch. City will have long spells of possession so I would like to see pace in attacking areas.
"I would like to see Rashford in. It's a big dilemma (with Lukaku). There is a lot more fluidity with this front three.
"That would frighten Manchester City."