After the 1994 Double winners and unparalleled Treble campaign five years later, United are once again at the peak of their powers, with only domestic rivals Chelsea standing between them and another Champions League crown.
"You have fear of many things but sometimes that drives you on," said Ferguson.
"I felt it was an important time for us when we started to make another change and began to rebuild again.
"It is not easy to manage change and when you see a team starting to break up due to age, injuries or merely because others are not maintaining the level they should be at.
"But bringing in these young players over last two or three years gives you optimism you can do it again. If I didn't think like that with players like Rooney, Ronaldo, Giggs and Scholes then I really am a pessimist."