Former player Paul Ince described retiring Manchester United boss Sir Alex Ferguson as one of a kind and also admitted finding a replacement would not be easy.
When Ince joined United from West Ham in 1989, Ferguson made sure his move was not ended because of a problem with his medical.
Blackpool manager Ince told Sky Sports News: "He's done the lot, you will never see anyone of his kind again. 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."
He added: "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, and he added: "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."
Professional Footballers' Association (PFA) chief executive Gordon Taylor said Ferguson would be "the toughest act to follow", and he told Press Association Sport: "The game of football will be a lot poorer place without him. He has been quite simply the best. He followed in Sir Matt Busby's footsteps and even surpassed him. He will be also be the toughest act to follow."
Taylor has been PFA chief throughout Ferguson's time at United and he admitted they had clashed on occasion - but that it was soon forgotten.
He added: "I will miss him - he has been a very good friend of the PFA throughout his career. Of course at times it has not always been smooth and we have had a difference of opinion but we always respected each other and we have had a lot more agreements than disagreements."