Rio Ferdinand hailed Alex Ferguson's "work ethic" after the Manchester United manager announced he was retiring at the end of the season.
"The bosses (sic) work ethic, his desire to win + to make us better players were unrivalled. Thanks boss," said United defender Ferdinand on Twitter as he became one of the first of the current crop of Old Trafford stars to salute Ferguson.
Earlier, David Beckham said Ferguson had been like a father to him.
"As I have said many times before the boss wasn't just the greatest and best manager I ever played under, he was also a father figure to me from the moment I arrived at the club at the age of 11 until the day I left," said Beckham, now with French giants Paris Saint-Germain.
Gary Neville, who along with Beckham and Paul Scholes was one of 'Fergie's fledglings', who graduated into the first team from the United youth system revitalised by Ferguson, said he was proud to have worked with the "greatest manager of our time".
Bobby Charlton, one of the United directors who appointed Ferguson in 1986 and who persuaded the club's board to stick with the manager when results were going against him four years later, said Ferguson's dedication had been exceptional.
"He would get up in the middle of the night and travel 300 miles if he thought there was a schoolboy he could sign. He loves the game and we at the club have had nothing to do really," said Charlton, United's captain when they won the European Cup for the first time in 1968.
FIFA president Sepp Blatter said the sport may never again see a reign like Ferguson's at United that spanned 26 years and an astounding 38 trophies, including 13 Premier League titles and two Champions League triumphs.
"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?"
Ottmar Hitzfeld, manager of the Bayern Munich side beaten in dramatic fashion by United in the 1999 Champions League final, said Ferguson's successor "will automatically be measured on Ferguson's success".
"The new coach has to be successful from the start or else it will be difficult for him," Hitzfeld told German Sky Sports News.
British Prime Minister David Cameron, a supporter 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."
But Alex Salmond, Scotland's First Minister, said his fellow Scot's record at his previous club Aberdeen -- rather than the success he had at United -- was the real proof that he was an all-time great.
While at Aberdeen, Ferguson won the Scottish league three times, in 1980, 1984 and 1985, breaking the dominance of Glasgow giants Celtic and Rangers, who between them have monopolised the title ever since.
"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," Salmond told AFP in an interview.
"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."
Current Scotland manager Gordon Strachan had the unusual experience of playing under Ferguson at both Aberdeen and United, as well as when he was in caretaker charge of the national side at the 1986 World Cup in Mexico following the death of Jock Stein.
"It really is the end of an era. Sir Alex Ferguson is a force of nature in world football and it truly was an honour to be managed by and to work with the best manager ever," said Strachan.
"We had some fantastic times together and it is perhaps fitting that he has made his decision to step down almost 30 years to the day since winning the European Cup-Winners' Cup with Aberdeen in Gothenburg.
"His achievements speak for themselves. Any time I am feeling down, I have a chuckle about our times together. I have probably experienced every emotion possible with Sir Alex but I wouldn't have missed those times for the world."