Uefa director of communications William Gaillard said: "These plans would turn the competition into a bit of a joke.
"Are United saying they want a static competition where the same clubs reach the latter stages every season?"
He added: "If that's the case, supporters and television companies would lose interest straightaway because the competition would become boring.
"The reason why the Champions League is so lucrative to clubs is because the competition is so exciting and full of surprises."