The Portuguese winger was signed by Ferguson from Sporting Lisbon in 2003 before the Scot sold him to Real Madrid six years later for a record £80m fee.
"Cristiano was the most gifted player I managed," he wrote in his new autobiography. "He surpassed all the other great ones I coached at United - and I had many.
"We helped Ronaldo to be the player he was and he helped us recapture the excitement and self-expression of Manchester United teams."
Ferguson called time on his career at the end of last season after a glittering 39 years in management.