The Real Madrid star experienced a good relationship with Ferguson at Old Trafford before his move to Real Madrid in 2009.
But, the Scot aside, Ronaldo claims that, though he is on speaking terms with Manuel Pellegrini and Carlo Ancelotti, he has not stayed close with many of his past coaches.
"The only friendship that I have with my former coaches is with Ferguson, with no one more," he told TVI.
"I had a good relationship with Pellegrini, and Ancelotti also, with whom I get along very well.
"I'm not in football to have friends. I am here to succeed."
The 29-year-old also commented on ex-Madrid coach Jose Mourinho – who is currently in charge of Chelsea – acknowledging that there was a "bad atmosphere" at the Bernabeu under the Portuguese and that his tenure was "a difficult phase in personal terms and the club".
"There were situations that were happening with other players," Ronaldo noted. "Also with the fans."
The forward will miss the start of Portugal's Euro 2016 qualification campaign due to concerns over his fitness following a series of thigh and knee problems that he suffered at the end of last season.