Sir Alex Ferguson believes he is a "phenomenon" among managers and his record of 25 years at one club will never be equalled.

The Manchester United manager admits he has had to change as he has got older, delegating more and toning down his approach with modern players who are not as tough-skinned as they were when Ferguson started out in the game.

Asked in a question-and-answer session with the New York Times earlier this week whether any manager was likely to stay at the same club for 25 years, Ferguson replied: "I don't think so, I'm a phenomenon."

He added: "There's no question about that. It's a more fragile human being that I am dealing with today than 25 years ago. They are cocooned by modern ideas, modern parents, modern agents, and they are cocooned by their own image at times.

"It's a different world for me, so I have to change myself to adapt to that. I have changed because of these things.

"One thing I've learned in the last decade is delegation. In the early days, I was involved with scouting, coaching, youth everything. You can't do that for a long period of time."

Despite earning considerably less than some of his young charges and reducing his workload, Ferguson left no doubt who was in charge at Old Trafford.

"You have to remember that the most important person at Manchester United is the manager," he said. "The minute a footballer becomes more important than the manager, your club is dead. The history of the club goes down the drain. I am the most important man at Manchester United. It has to be that way."

Ferguson said he had a "great relationship" with the Glazer family, whose takeover and running of United has alienated many supporters but who appear to have a hands-off approach where their manager is concerned.

"They never bother me. They never ask questions, they never phone me and they never interfere with my job. I am in a privileged position," he said.

The 70-year-old gave the impression that he would have to be carried out of United because picking up his pension was not under consideration. "Retirement is for young people, not older people," he said. "Young people can do something else. When you're older and you've been on that treadmill for [the] length of time I've been on it – if I get off the treadmill, where do you think I am going? Down there. Trust me. When you get older, don't retire."

Ferguson said he enjoyed the "fantastic" cultural diversity in the English game, even if he finds the late hours kept by his Brazilian players a nightmare – "that's their culture and I'm trying to change that". The Scot also expressed sympathy for the England squad and the pressures they operate under.

"I think the expectation of England is too huge. It's too big. You've got to also look at the programme England players are faced with going into a World Cup … the league is unforgiving, and it's very difficult.

"We've been in a lot of finals in the last decade. That definitely takes a lot of energy out of you, and it's a big problem in the English game."

Meanwhile, Wayne Rooney is rated a serious doubt for United's fourth-round FA Cup tie at Anfield on Saturday. The England striker has not trained this week after picking up a leg injury during the 2-1 win at Arsenal last weekend.

Phil Jones, carried off at the Emirates, is almost certain to miss the Liverpool game and Rio Ferdinand will probably be judged not fit enough.

Nani has undergone a scan on a foot injury picked up on Sunday and he is expected to be sidelined for some time.