i'm not saying that the youth isn't up to standard, but
defending requires a certain calmness and experience that the youth
generally lack, especially for central defence.
also, it's always been taught that great championship winning teams
are built on two principles, a string defence, and a strong
spine.
United's defence is strong, save for the full-back positions at
times. the rationale for a strong defence is that the attackers can
pour foward safe in the knowledege that if they lose the ball, they
dun have to track back to win it back. it keeps them focused on
attacking, making it seem like attack is the best form of
attack.
as for the spine, United have a spine that's been in place for
about 2 seasons, so Fergie's just beefing it up. he does lack a
focal point of a centre foward though, so even if he does move for
Saviola (free), he'll need someone big and strong fro the crosses
and holding up play, etc. other than that, the spine shouldn't need
any tinkering with.