Who in particular do you think people have overated?
Just remember a lot of them are part of the treble team, and for those who ain't - Adriano (early days), Ibra, Martins, Crespo, Cruz, Recoba, Maxwell, Figo (first & sec year),
Vieira (injured like Vidal and Sanchez these days),
Dacourt (as a backup), if you ask me what the team was lacking of, that would probably be a true No.10, hence S10 fitted like a glove and play so well starting form the first game, cause the whole team was built on 4-3-1-2 where the 1 was the only missing piece.
I know there are some that wasn't that great (Old Bobo, Cesear?(winger), WDM, Veron (rebuild time?)), hence the missing playmaker problem, but think about the depth and the overall quality.
P.S. I like Mancini, I don't get the hate for him either, but he wasn't who he is today, and head on giant and knock out stage require something he didn't have back then.