Ferguson tells the Guardian that he can see Ronaldo back at United if it doesn't work out in Madrid.
"His best is yet to come because he is just 24, I will be very proud when I see [him at his best] because we had a part to play in it. He came to the right club at the right part of his career. He could have gone to Real Madrid or Barcelona as a boy but he came to the right club and he appreciates that. And he may come back, you never know.
"I don't see it happening if things go wrong at Real Madrid necessarily, but he loved Manchester United so it may happen. He loved the club. If you look at the six years he had with us, he never missed training and he was never injured. He always played. He was fantastic.
"There is no doubt it is a huge loss. It is a challenge for us because it won't be the same without Ronaldo. He is the best player in this world of ours - by absolutely miles. He is streets ahead of Lionel Messi, streets ahead of Kaka. Look at his figures. They are unbelievable."
He also went onto say, "We tried for Benzema because, at 21, I felt there would be an improvement there. He is tough, with a good physique and goalscoring record, so it was worth going a wee bit extra for him because of his age. But when it went to 42m euros, it was beyond his value. We went to 35m and I think that was fair.
"We explored signing Franck Ribery in a nice, quiet way when David Gill was at one of his meetings with Bayern Munich in March. They told him: 'Absolutely no chance' and the one thing you can say about Bayern is that when they say something they mean it."