Serie A champions Juventus are reported to have made a bid of €100 million (£88m/$116m) for Ronaldo, who is Madrid's all-time leading goalscorer after nine seasons at Santiago Bernabeu.
Moggi, who headed up Juve's board from 1994 until 2006 when he was a central figure in the Italian football scandal that saw him given a life ban from football, was initially sceptical of the deal going through, but has since claimed the transfer was already at an advanced stage.
The 80-year-old told Italian TV network Tele7Gold : "In my opinion, he has already signed and passed the medical examination with Juventus in Munich.
"This is what I think after talking to important people."
Moggi recalled his attempts to make Ronaldo a Juventus player in 2002, when he was emerging from the Sporting CP youth ranks and becoming one of the Primeira Liga's hottest talents.
A proposed transfer involving Chilean striker Marcelo Salas moving in the opposite direction never transpired, according to Moggi, and Ronaldo joined Manchester United in 2003.
"I received an alert," said the former Juve chief. "I sent an observer to look at him, he was an 18-year-old boy, but he impressed me with his maturity and his choices.
"In the middle of the day I made the contract. Unfortunately, Marcelo Salas refused to sign for Sporting Lisbon because he wanted to return to River Plate.
"I offered five million more for an 18-year-old player, but it could not be done at that time."
The prospect of the Portugal star joining up with the Turin giants has been talked up by potential team-mate Blaise Matuidi , who said: "It would be great to have one of the best players in the world with Juventus in the same club that I am."
And the idea of the 33-year-old reuniting with former Madrid colleague Gonzalo Higuain has the latter's brother and agent excited, as he believes they would be "fantastic" together .