Cristiano Ronaldo admits Sir Alex Ferguson is the main reason he has decided to return to Manchester United.

Ronaldo posted a message to the club on social media on Tuesday, saying, 'Sir Alex, this one is for you…'

And in his first interview as a United player, the Portuguese went into more depth over his relationship with the legendary manager.

Ferguson is the manager who bought him from Sporting Lisbon in 2003 for his first spell at United and stepped in again last week to ensure Ronaldo joined United ahead of cross-town rivals Manchester City.

"As everybody knows, since I signed at 18, Sir Alex Ferguson was the key," he told MUTV.

"I remember when we played against Manchester when I was at Sporting Lisbon. For me, Sir Alex Ferguson is like a father in football for me. He helped me a lot, he taught me many things, and in my opinion of course he had a big role because the relationship that we had, we keep in touch all the time, and he's an unbelievable person."

"I really like him a lot and he was the main key for me to be in the position that I am, that I signed for Manchester United."

Ronaldo's agent Jorge Mendes held discussions with City last week and it was certain he was headed to the Etihad until Sir Alex Ferguson stepped in.

"I think it's the best decision that I have made. It's right on point in my opinion. I moved from Juve now to Manchester, it's a new chapter, I'm so happy and glad, and I want to carry on again, to make history, to try to help Manchester achieve great results, to win trophies." he said.

"As you know, I have a fantastic history with this amazing club. I was there at 18-years-old and of course I'm so happy to be back home after 12 years. So I'm really glad and looking forward to start my first game."

Ronaldo also talked about his relationship with Ole Gunnar Solskjaer.

"Well, we had a chat, but of course I'm going to have time to speak with him face to face, to know what he expects," he added.

"As you know, I played with him for two or three years at Manchester United so I have a good relationship with him but now with a different role, I'm a player and he's a coach."

"But it doesn't matter, my relationship with him is great and as I say before, I'm there in Manchester to help the team achieve his results and the coach can count on me for whatever he wants. So I'm available for everything."

Manchester United fans went into meltdown on learning that Ronaldo was headed back to Old Trafford and Ronaldo says he can't wait to see them soon.

"The fans is the key of football,' he said. 'Now, with all the stadiums full of supporters, it's not the same that we had one year ago, those empty stadiums. The fans, they are the key, and I'm so glad."

"The Manchester United fans, they are special, I know, I remember very well. I know they still sing my music which has made me feel even more happy."

"My commitment is to give everything on the pitch, like I did before, like I do it all the time, and try to do my best, help the team score goals, make assists, win games, and I hope to see them very, very soon."