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And 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, with the above picture attached, the following 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 for a world-record £80 million fee, but has always held the Manchester club - and Sir Alex in particular - in high regard.
Meanwhile, Ronaldo's Real Madrid team-mate
Sergio Ramos added his own tribute on Twitter: "Acknowledgement to an amazing career at Manchester United. Remarkable #SirFerguson, a football's legend."
David Moyes is expected to be announced as Manchester United's next manager, with the appointment set to be concluded by the end of this week and potentially evening by Wednesday evening.