While former players hailed his extraordinary record of winning trophies, former United midfielder Paul Ince warned that his successor would find Ferguson's shadow looming over them in his new role on the club's board.
Cameron, a fan of Premier League strugglers Aston Villa, tweeted: "Sir Alex Ferguson's achievement at#MUFC has been exceptional. Hopefully his retirement will make life a little easier for my team."
FIFA president Blatter said the football world may never again see a reign like Ferguson's at United that spanned 26 years and 38 trophies.
"Will his longevity at the top ever be repeated?"
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Peter Schmeichel, the Danish goalkeeper who was one of Ferguson's most important signings at United, described his decision to retire as a "bombshell" and said he "can't make sense of the timing".
"It has come as a bombshell, I really don't know what to make of it," Schmeichel said.
"Yesterday I was really happy with what he had done this season, now this, I'm disappointed and very sad.
"He's always said the day would come when something in his life wasn't right, is it is hip operation (he will have this summer)? I can't see him retiring over that. (Until) he explains why, we can guess and guess."
"Replacing Alex Ferguson is such a massive, massive job. Whoever goes in will need the help of Alex Ferguson," he said.
"Whoever comes in if it doesn't go well you've got Sir Alex Ferguson upstairs and it can put added pressure on you.