Real Madrid star Cristiano Ronaldo, who played for six seasons under Ferguson at Manchester United, tweeted his appreciation for the manager who made him a star.
He attached a picture of the pair when he signed in 2003.
Ronaldo joined Madrid in 2009 for a world record fee of around £80m.
Sir Alex Ferguson's brisk way with words has influenced the way we talk about football.
A timeline of the life of outgoing Manchester United manager Sir Alex Ferguson.
A special photo gallery of Sir Alex Ferguson's career at Manchester United as he steps down from his post as manager of the club.