The Real Madrid striker, 26, left Old Trafford for the Bernabeu in a transfer deal worth a world record €94 million in the summer of 2009.
The Portuguese international still has fond feelings towards his former club, and compared his current coach Jose Mourinho to Sir Alex, while revealing that he misses playing football in the Premier League.
“Sir Alex Ferguson is my second dad – he helped me a lot to improve and now Mr Mourinho too because they are experienced managers who win many things, so for me they are the best coaches,” he told Sky Sports.
“I have good memories from Manchester and when I watch sometimes the games I miss it a lot because it's a part of me I left in England.
“Just because I play in Madrid I'm not going to miss speaking with the old guys, so when I have an opportunity I speak with Sir Alex Ferguson.
“It was important for me when I played there, when my life was there so it is good to speak with him because I'll never forget who really helped me.
“There is a beginning and end to everything – what I did in Manchester is completely finished because I won almost everything there.
“I always try for a new challenge. To move from Manchester to Madrid is not about the contract – it is about a new challenge, being close to my family, different weather, the language.
“The most important reason why I moved is the challenge because I really like to put myself under pressure to see what I can do.
“But I really miss the Premier League - for me it is along with the Spanish league the best league in the world.”
Last season Ronaldo scored an impressive 54 goals for Real Madrid and has started the 2011-12 campaign in a similarly threatening fashion, bagging a hat-trick in a 6-0 rout of Real Zaragoza.
The 2008 Ballon D’Or winner has refused to rule out a move back to the Red Devils – or any Premier League club – but says he is happy in the Spanish capital at the moment.
He said: “I don't close the door, so maybe in the future I hope so, because I know the league, I know the players, I know the atmosphere.
“Why not? I hope so one day, but you never know. The team that I like more in England is Manchester but I cannot say that I am going to play there one day because I am very, very happy in Madrid.
“I've said a few times that, if Madrid want, I can sign for 10 years more until the end of my career but nobody knows the future.
“I cannot lie to you and say I will stay here 10 years, that I am not going to come back to Manchester. It is not fair on my part. Let's see what future God has prepared for me.”