The Portuguese spent six years at Old Trafford before moving to Real Madrid in 2009 where he has continued to enjoy success both individually and as part of the team that last year wrested the Primera Division title away from Barcelona.
The 27-year-old became the most expensive footballer in history when he moved to the Bernabeu for £80million and Ferguson, while acknowledging he would welcome Ronaldo back with open arms, knows he is beyond United's reach.
Asked if the door was always open for the player, Ferguson told a number of national newspapers: "I'd like to think it would be but I don't think it will happen.
"You're talking about incredible amounts of money now.
"What would you think his value would be? He's definitely unbuyable.
"Who could afford to buy him? There's talk of the Russians, perhaps. But do you think he would go to Russia?
"He complained about the weather here, let alone the middle of winter in Russia.
"He was definitely good value for Madrid but he always wanted to play for them, it was his boyhood dream."
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