United's acceptance of Madrid's £80 million bid for the Portugal international earlier this week brought an end to one of the longest-running transfer sagas of recent years.
Ronaldo's desire to leave the reigning Premier League champions had become all too apparent in recent months, and Ferguson insists it was out of the question to expect the player to stay any longer.
"He wanted to leave, it's as simple as that," Ferguson told The Mail on Sunday.
"He was going to go some time. We've done well to keep him for so long."
Madrid's record swoop for the Fifa World Player of the Year is set to be concluded by June 30.
On top of the £80m Madrid president Florentino Perez has spent on Ronaldo, £56m was paid to AC Milan for Brazilian striker Kaka two weeks ago.
The club's spending spree this summer comes as Perez tries to re-establish Madrid's 'Galacticos era', and according to former player Zinedine Zidane, Franck Ribery is next on the hit list.
"He's a magnificent player who would fit in at Real," the 36-year-old said. "And playing in this team is a huge opportunity in the career of a player, which I don't think you could refuse.
"After Munich, it would be a beautiful opportunity for Franck. The decision is up to him."
Addressing his own career, Zidane said he had clearly defined plans for the future.
"Currently, things are going well with Florentino. He took me into his team because I know the Real changing room well," he said. "For the time being, I'm there to learn, to see how things work outside the changing room.
"Next year, we'll see what might interest me, in order to find something more precise at the club.
"I think I could really become part of the management team at the club and maybe take charge of its destiny."