"You have to prepare for the future and we're always looking for young centre-backs," said Ferguson. "Sometimes we develop them, like Gerard Pique, Jonny Evans or Ryan Shawcross, who we sold to Stoke some time ago.
"You're always looking for young centre-backs because it's such an important position in terms of the spine of your team, which needs to be strong.
"We identified Chris Smalling and Phil Jones over the past year and they've done exceptionally well, both of them, and of course Jonny Evans grew up at the club. He's been with us since he was 12-years-old.
"It's interesting because, years ago, you used to get plenty of centre-backs, they were falling off trees. But for some reason, over the last 10 years, it's become more difficult to get really top centre-backs.
"You don't know why that's the case, but it's an interesting fact that you just can't get the centre-backs you were able to get.
"There's more emphasis now on attacking play from all clubs, which means there's less concentration on the really top defending you used to get, particularly in Italy, with players like [Claudio] Gentile, who could man-mark.
"You don't get man-marking these days, it's all zonal defending, so it's a big change that way."