Ronaldo was linked with a move back to the club where he spent six years after signing from Sporting Lisbon for £12.25m in 2003 before moving to Madrid in a then world record £80m deal.
But the Portuguese signed a new deal with Real in September last year to end speculation over a return to the Premier League.
"He was one of the most dialled people on my phone at the time. As footballers you keep your cards close to your chest," Ferdinand told the Sun.
"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 view - but no one will ever know.
"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."