The 70-year-old celebrated 25 years at the Old Trafford helm in November 2011.
"You have to remember that the most important person at Manchester United is the manager," he told the New York Times. "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."
The Scot also revealed that he has had to adapt to how footballers have changed since he took charge in 1986.
"I have certainly mellowed," 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."
The Red Devils travel to Anfield to face arch rivals Liverpool in the FA Cup fourth round in Saturday.