Sports writer Mihir Bose said he was surprised by the timing of Ferguson's decision, despite his upcoming surgery, saying he had expected him to carry on as manager for another couple of years.
Ferguson -- who is seen as a kind of father figure by some of the young players he developed -- has "that ability to reach out to people," said Bose, but he is also a skilled political operator.
"He could be delightful but he was a man who made sure that he controlled the agenda," he told CNN. "If you crossed his path he made it very clear that he controlled everything at Manchester United."
Those who got the famous "hairdryer treatment" -- loud shouting directly in someone's face -- included many sports journalists over the years. Ferguson would also refuse to speak to reporters if he didn't like what they said.
Despite that thorny relationship, Ferguson will be remembered as an iconic figure in English football history, said Bose. The big challenge now is how Manchester United will manage the transition to ensure continued success, he added.
'A sad day'
Former Manchester United and Denmark goalkeeper Peter Schmeichel told CNN he was still trying to make sense of the news.
"It is a sad day. I'm shocked, I'm sad, I'm disappointed. It's a day I think everyone who loves Manchester United, everyone who's worked with Sir Alex -- it's a day that we've been expecting, but I have to be honest I didn't think it would be now -- I thought it would be a couple of years down the line."
Schmeichel said he was certain that Ferguson had not been forced out but had made the decision himself, given his record of success and changes already happening on the club's board this summer.
He paid tribute to Ferguson's knowledge, skills and philosophy as a manager, particularly his ability to bring on young players.
Internationally known footballers like David Beckham, Ryan Giggs, Paul Scholes, Gary Neville and Nicky Butt were all created by Ferguson, Schmeichel said.
"He is the best manager in the world and he's the best guy as well. He's a really good friend as well."
Ferguson's talent has been in managing players individually rather than as a group, Schmeichel said.
"The end result is that everyone plays really well for the team," Schmeichel said. "Yes, he can be hard, he can be tough. ... In a way he becomes your second dad. He has to educate you in life, he has to prepare you for what comes next in life after football."
'Vision, energy, ability'
David Gill, who will step down as chief executive of Manchester United in June, said it had been a "tremendous pleasure" to work alongside Ferguson over the past 16 years.
"We knew that his retirement would come one day and we both have been planning for it by ensuring the quality of the squad and club structures are in first class condition," he said.
"Alex's vision, energy and ability have built teams -- both on and off the pitch -- that his successor can count on as among the best and most loyal in world sport."
Ferguson began his career on the soccer pitch, playing for Scottish clubs Queen's Park, St. Johnstone, Dunfermline, Glasgow Rangers, Falkirk and Ayr United.
But it was when he returned to the game as a manager, working at East Stirlingshire, St. Mirren and then Aberdeen that people really began to take notice.
He led Aberdeen to three Scottish titles, four Scottish cups, one League Cup and one European Cup Winners' Cup before moving to Manchester United in November 1986 following the dismissal of former manager Ron Atkinson.
It didn't take long for the Scotsman to start turning things round at a club that was then near the bottom of the league.
Since then, Ferguson has dominated the English game -- his string of victories making him, according to the Manchester United website, "the most successful manager in British football history."