The Portuguese had been a reported target for United, for whom he enjoyed a six-year spell between 2003 and 2009, collecting three Premier League titles and the UEFA Champions League in a trophy-laden stint.
But the 28-year-old ended speculation on his long-term future at the Bernabeu by signing a new five-year contract in September and has gone on to score 20 La Liga goals in just 17 appearances so far this term.
Ferdinand believes the opportunity to bring the Portugal international back to the club has now been missed, but believes United would have been his destination of choice had he decided to leave Real.
When asked by The Sun if he had contacted Ronaldo, he said: "What do you think? He was one of the most dialled people on my phone at the time.
"But I think the opportunity has gone now. If we could have done it, I think it would have been last summer. He's signed a new deal at Real now.
"As footballers you don't really let on too much to each other. It's like a chess game. And in those situations you have got your one and two first-choice moves or scenarios that you want to happen.
"So to let people know is sort of letting the cat out of the bag. You play your cards close to your chest.
"But if he was going to leave Real Madrid, I think Manchester United would have been the place he would have come to. That's my personal view, but no-one will ever know now."
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