Fergie said: "It's all very well having all the money but you can't buy everyone. It was proved in the summer.
"We didn't want to sell Ronaldo, other clubs don't want to sell their best players, and there are only so many players you can buy."
The Old Trafford boss, who has clocked up 21 years as United manager, told the club's official magazine, Inside United: "Back in the 1950's Sunderland were called 'The Bank of England Team' - and they got relegated. There are no guarantees.
"You can buy 11 individuals - you can buy 11 Robinhos but it wouldn't be a team would it?
"The art of management is building a team with balance, with certain characteristics that blend with each other - that's the most important thing."