Ronaldo signed for United in 2003 and left for Real Madrid six years later for a world record £80 million fee, and he says the time he spent at Old Trafford was instrumental in his development.
The Portuguese striker was disappointed not be able to attend the dinner held to mark Ferguson's 25 years in charge of United but called David Gill to pass on his regards.
"I wanted to go the party. It was amazing for Sir Alex. He's had such an amazing career and he keeps going. Because of our training schedules I couldn't make it. That's why I was desperate to speak to him on the phone," Ronaldo was quoted as saying by the Sunday Mirror.
"Unfortunately I couldn't get through so I told chief executive David Gill to pass on my congratulations for all Sir Alex has achieved. He is a fantastic manager. He helped me so much in my career.
"Sir Alex has a special place in my life. In fact, he was the main man. I was not famous, I was not a star. I arrived at Old Trafford as just another young talent. He was the one who told me to do all the right things. He gave me the opportunity to play in one of the biggest clubs in the world. So he is one of the most important people in the world for me."
Ronaldo is now coached by Jose Mourinho at Real Madrid - the man many have tipped to take over from Ferguson, when he finally retires. Ronaldo says he rates both as the best in the world.
He said: "You can't tell the difference. They are incomparable. They have different ideas, different opinions. For me they are both the best in the world. I worked with Sir Alex for a few years and I know he deserves everything that he has achieved in his career. He works so hard, he is clever, he has experience, he is a human guy.
"Mourinho is similar, but he is younger. I have worked for a while with Mourinho now too so I'm entitled to say they are both very special. It's not just Mourinho who's special. They're both special! This is why I like them both so much."
Meanwhile, Ferguson says he was stunned when it was announced the North Stand at Old Trafford was to be named in his honour prior to kick off against Sunderland.
He said: "I was emotional when David Gill announced it to the fans before the game. I didn't have a clue the club was planning to do that. It's a fantastic honour. The occasion got to the players and it was a long day for me as well. The last 15 minutes went on forever and Sunderland threw everything at us."