Ronaldo was linked with a move back to the club with whom he spent six years after signing from Sporting Lisbon for £12.24million in 2003 before moving to the Spanish giants for a world record £80million transfer.
But the Portuguese signed a new deal with Real in September last year to end any speculation over his future.
Ferdinand revealed he was in constant contact with his former team-mate beforehand, but admits United's best chance to re-sign Ronaldo has gone.
"He was one of the most dialled people on my phone at the time. As footballers you keep your cards close to your chest," said the former England international to 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."