British Prime Minister David Cameron and world football chief Sepp Blatter led the tributes on Wednesday after Manchester United manager Alex Ferguson announced he was standing down at the end of the season.
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.
"His achievements in the game place him without doubt as one of the 'greats'" Blatter said on Twitter.
"Will his longevity at the top ever be repeated?"
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."
Paul Ince, who was once the engine of United's midfield, said succeeding Ferguson at England's most successful club could be a poisoned chalice.
"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."
Alex Salmond, Scotland's First Minister, said Ferguson's trophy haul at his previous club Aberdeen -- rather than the success he had at United -- proved he was an all-time great.
While at Aberdeen between 1978 and 1986, Ferguson managed to break the dominance of Glasgow giants Celtic and Rangers, who between then have monopolised the title of Scottish champions ever since.
"He's had a fantastic career," Salmond told AFP. "I'm pleased he's managed to catch everyone by surprise because I know he'll be pleased!"
"I still think the greatest test of a real football manager is the ability to win the biggest prizes with unfashionable sides or less powerful teams.
"I still think that winning the European Cup Winners' Cup with Aberdeen in Gothenburg (in 1983) is the indication that what we're talking about here is one of the all-time great football managers."
Michael Owen, the former England striker who was used sparingly by Ferguson in two seasons at United, said he was honoured to have played under him.
"It's just not sinking in! Manchester United with no Sir Alex just doesn't feel right," he said.
"What a man and great that it has ended on his terms. What a privilege to have played under arguably the best manager the world has ever seen. His record will surely never be eclipsed.."
Even younger players who Ferguson decided did not make the grade at Old Trafford were quick to praise their old boss.
Former United forward Giuseppe Rossi, now at Fiorentina in Italy, tweeted: "What a career! Proud and honored to have been coached by Sir Alex Ferguson. #thankyougaffer."