The Scot has overseen an unprecedented level of success in his 26 years at Old Trafford, helping the club surpass Liverpool's record of top-flight titles and take their overall tally to 20.
And current number two Mike Phelan believes the 71-year-old's desire to win is a major factor in his success.
He told BBC Radio Five Live's Sportsweek programme: "He's maintained the desire to win, you can never take that away from him and what he has done over a period of time is he's mellowed.
"He's passed on certain responsibilities to individual people, he lets you work, he lets you get on with the job, he expects honesty and trust from you and I think he gets that."
He added: "His biggest strength is his leadership.
"He's put things in place over many many years and he's had that desire to win.
"And when you've got someone who's pushing and pushing and pushing it brings along a lot of people and if you're in that same frame of mind and you've got the same work ethic that he's got, then it can only be a benefit to everybody.
"He's definitely produced some excellent teams, some excellent players and I think it's been backed up by good quality staff.
"He's matured like a good bottle of red wine. He's got better and better with age and he's still going strong."
Former England boss Steve McClaren replaced Brian Kidd as Ferguson's assistant in 1999 and was on the staff at United when they won the treble.
He stayed at Old Trafford until 2001, winning two more league titles before moving into management on his own with Middlesbrough.
He has only tasted championship glory once since, winning the Eredivisie with FC Twente, but he is in no doubt as to the reason United keep piling up the titles.
"It's solely down to the manager," he said.
"What he's created at Manchester United, the kind of culture and environment. It's the winning mentality that he has."