The Scot says Ronaldo's desire to make a 2009 dream move to Real Madrid led him to allow the star forward to leave when the LaLiga giants made their £80million bid for him.
Ferguson said on the UTD Podcast: "I was not against that, six years was great for a boy from Madeira, getting him here just after his 18th birthday.
"I think we did well that way and it gave me a chance to get a replacement.
“We got Antonio Valencia and he was fantastic so we were very lucky that way.
"I knew (how good he would be). When he got to that kind of platform at Real Madrid.
“To be fair to Real, they are one club that has produced world-class players for years and years and won the European cup so many times, 13 or something.
“You've got to give them credit - it is a platform where any great player would love to go to."
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