Former player Paul Ince described Ferguson as one of a kind and also admitted finding a replacement would not be easy.
Blackpool manager Ince told Sky Sports News: "He's done the lot, you will never see anyone of his kind again."
When Ince joined United from West Ham in 1989 Ferguson made sure his move was not ended because of a problem with his medical.
"I remember the first day that I joined Man Utd. I failed my medical and I thought my move to Man Utd was going to collapse. The way he treated me was like a son and I will never forget that moment.
"To play under that man was so demanding, his standards were so high. We had our ups and downs, a lot have ups and downs with him."
Ince said Ferguson's replacement would have a difficult act to follow.
"It's got to be someone with a massive character. It has got to be someone who can deal with what it takes to be a Manchester United manager.
"Whoever comes in to replace him is going to have to deal with the Man United legacy."
Ince believes Ferguson staying at the club as a director could be a double edged sword for his successor.
"It can have its advantages and disadvantages. 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."