Ferdinand normally stays back, and reads the game, thats his way of defending i see nothing wrong with that.
I believe you are underrating both Evra and Rio. The former being one of the top leftbacks in Europe, and is a better defender than he gets credit for, he leaves a lot of gaps behind, but when he's in position he's a tough defender to get past imo.
Rio is one of the best defenders in Europe, theres no doubting that if you ask me, he reads the game very well, he's a physical presence and is brilliant in the air. But Man Utd's defense isnt about individuals, imo Fergie has got them to be a very strong unit that is almost impossible to get past.