United entertain Champions League opponents Rangers this week.
Fletcher told the Scottish Sun: "They would be massive clubs down here if it came to it.
"I don't know the ins and outs of it, I've only read and heard little bits of what's been said.
"Given a little of time the power and the fanbase - on top of how big the clubs are - would definitely see them do well in England and relish the opportunity.
"I think people down south know that.
"Those people who say they would struggle just say that to try and wind us Scots up. Simple as that."