Sir Alex Ferguson retires as Manchester United Manager after 27 years
Ferguson, 71, will step down as United manager at the end of the season after 26 years in charge in which he won the Premier League title 13 times.
"It is unbelievable to change around probably four different squads and have the success he has," Robson said.
Prime Minister David Cameron described Ferguson's record as "exceptional".
Mr Cameron, an Aston Villa supporter, added: "Hopefully his retirement will make life a little easier for my team."
“
It's come as an absolute bombshell. I'm sad and disappointed
”
Former Manchester United player Peter Schmeichel on Sir Alex Ferguson's decision to retire
Former Newcastle and England striker Alan Shearer, who almost signed for Manchester United from Blackburn in 1996, said: "If it wasn't for my love of Newcastle then I would've had to sign for Sir Alex. I was that close I'd actually found a house in Manchester.
"His knowhow, his desire, his hunger, his will to win and longevity are absolutely staggering.
"When you saw him last season squaring up to Mancini, at his age you have to admire that. He's an absolute genius. If you could bottle that it'd be worth a fortune."
Labour leader Ed Miliband tweeted: "Proud man. Great manager. Staunch Labour Party supporter. Sir Alex Ferguson will never be forgotten."
Ferguson has won 38 trophies in total since taking over from Ron Atkinson on 6 November 1986, including this season's Premier League crown.
A host of former United players joined Robson, who was at United from 1981 until 1994, in paying tribute to Ferguson.
Sir Bobby Charlton spoke fondly of a "fantastic" and "sensational" manager.
"I am a director at United but I hardly do anything because we are winning all the time and it is all down to Sir Alex Ferguson," said Charlton, who made more than 600 appearances for United between 1956 and 1973.
"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."
Ferguson's Man Utd trophies
Premier League: 1993, 1994, 1996, 1997, 1999, 2000, 2001, 2003, 2007, 2008, 2009, 2011, 2013.
FA Cup: 1990, 1994, 1996, 1999, 2004
League Cup: 1992, 2006, 2009, 2010
Champions League: 1999, 2008
Cup Winners Cup: 1991
Fifa Club World Cup: 2008
Uefa Super Cup: 1992
Inter-Continental Cup: 1999
FA Charity/Community Shield: 1990 (shared), 1993, 1994, 1996, 1997, 2003, 2007, 2008, 2010, 2011
Former England captain Paul Ince, who played under Ferguson for six years between 1989-1995, does not believe there will be another manager who will replicate Ferguson's achievements.
"You will never see anyone of his kind again," Ince said.
"His standards were so high. He was so demanding. Yes, we had our ups and down. The way he treated me was like a son."
Champions League winning goalkeeper Peter Schmeichel said he could not make sense of the timing of the announcement.
"It's come as an absolute bombshell. I'm sad and disappointed," he said.
"I was really, really hoping he was going to stay for another couple of years."
But Dwight Yorke, Schmeichel's treble-winning team-mate of 1999, said he could understand why Ferguson has decided now is the time to retire.
"Regaining the Premier League this season, he's managed to do that," Yorke added.
"So in many respects that's the reason, with the hip operation and David Gill going, I just feel it's the right time for him to go."
Former England striker Michael Owen , who played under Ferguson at United from 2009 until 2012, said he was proud to have worked with the Scot.
"He is arguably the greatest manager to have ever lived and to say I played under him for three years is a proud thing to say," he said.
Real Madrid winger Cristiano Ronaldo , who played for United for six years from 2003, simply tweeted: "thanks for everything, Boss" accompanied by a picture of the two together, while former United striker Ruud van Nistelrooy tweeted: "2001-2006, 219 games, 150 goals under the most successful manager in football history. It was a unique privilege."
Ole Gunnar Solskjaer , current manager of Norwegian side Molde, spent over a decade at United, playing over 200 games from 1996 until 2007.
He told MUTV: 'I will never forget the loyalty he showed me. Everything I have learnt I have learnt from the boss'
England manager Roy Hodgson described Ferguson's announcement as "a sad day for English football".
Tributes from outside of football
England rugby union coach Stuart Lancaster: "His longevity and what he has achieved as a coach I think is unparalleled in world sport. I admire him hugely for what he has done."
Golfer Rory McIlroy: "An end of an era today! Sir Alex Ferguson, the greatest of all time! United will have a tough time trying to replace him!"
"It marks the end of an era in football management," Hodgson added. "No one will be able to match his achievements, his dedication, his support for colleagues in need and his team building know-how."
Senior figures at football governing bodies spoke highly of Ferguson's contribution to the sport as a whole.
Fifa president Sepp Blatter tweeted: "His achievements in the game place him without doubt as one of the 'greats'.
"It was an honour to present Sir Alex with award at 2011 Ballon D'Or. Will his longevity at the top ever be repeated?"
Uefa president Michel Platini described Ferguson as a "visionary" who "has made a massive contribution to football across Europe", while Premier League chief executive Richard Scudamore said he defined the Premier League era.
"The Premier League has had the privilege to witness many great players, managers and teams," he said. "No one has made as great a contribution to the Premier League than Sir Alex Ferguson."
League Managers Association chairman Howard Wilkinson said: "He is the epitome of the mantra 'Survive, Win, Succeed'. But, in private, with those he trusted, he was the very best sort of friend you could ever wish for."
BBC
