Ronaldo, who has signed a two-year deal at Old Trafford, first emerged as a global star at the Premier League club, where he spent six years under Ferguson before joining Real Madrid in 2009.
Soon after completing his £12.86million move back to United, the 36-year-old signed off a message on Instagram by saying: “PS – Sir Alex, this one is for you…”
Ronaldo spoke of his “never ending love” for United and that he was “back where I belong”.
He wrote: “I can’t even start to explain my feelings right now, as I see my return to Old Trafford announced worldwide.
“It’s like a dream come true, after all the times that I went back to play against Man United, and even as an opponent, to have always felt such love and respect from the supporters in the stands. This is absolutely 100 per cent the stuff that dreams are made of!”
Ronaldo added: “I’m right here! I’m back where I belong! Let’s make it happen once again! PS – Sir Alex, this one is for you…”
On Friday it looked as if the 36-year-old could be heading for the blue half of Manchester as his exit from Juventus was announced, but a deal was quickly put in place to take him back to United.
The five time Ballon d’Or winner, whose two-year deal has the option of a third, scored 118 goals in six seasons for United between 2003 and 2009.
Manager Ole Gunnar Solskjaer told the club website: “You run out of words to describe Cristiano. He is not only a marvellous player, but also a great human being. To have the desire and the ability to play at the top level for such a long period requires a very special person.
“I have no doubt that he will continue to impress us all and his experience will be so vital for the younger players in the squad. Ronaldo’s return demonstrates the unique appeal of this club and I am absolutely delighted he is coming home to where it all started.”
During his first spell with United, Ronaldo won three Premier League titles, a Champions League, the FIFA Club World Cup, two League Cups and one FA Cup.
He went on to win four more Champions Leagues with Real Madrid as well as four league titles split equally between Real and Juventus.
His 451 goals for Real have him comfortably clear as the club’s all-time record goalscorer, while international honours arrived when Portugal – for whom he has a country-best 109 goals – won the 2016 European Championship.