The Portuguese star, who worked with Sir Alex at United for six years, has joined the likes of Paul Ince and David Beckham in thanking the 71-year-old for his mentoring during the important stages of his career.
Ronaldo posted on Twitter: "Thanks for everything, Boss."
The 28-year-old won three Premier League titles with the Red Devils, as well as lifting one Champions League trophy and one FA Cup.
Ronaldo left Old Trafford to join Real Madrid in 2009 but has always held the Manchester club - and Ferguson in particular - in high regard.
Everton manager David Moyes has emerged as the clear frontrunner to take over at United.