I think there are several reasons why he lost some form to be honest. Like an earlier poster pointed out, as soon as coaches, or other clubs spot a weakness, they will exploit it. Likewise, if you are particularly strong in one area, they will exploit it, but that same principle applies to every player. Everyone has weaknesses - no-one perfect. When he is on form, he is a quality player, and he has shown that. One v one he was devastating, and there weren't to many Full-Backs in the Premier League who were as strong all round as Valencia. Physically he is superb. He's big, he's strong, and he has pace. On the rare occasion a Full-Back could match him for pace you had to match him for strength, and the same goes the other way. With space in behind he could be devastating at times, and although you could argue that he could be slightly one dimensional, we have players in the squad capable of complementing that, and bringing the best out of him.
Football is a sport based on confidence. No two ways about that. Every top player has an 'ego' of some sort, and if you don't have that, and you don't have that self belief you are going to struggle. Obviously I don't know him personally, but he doesn't seem to have that confidence in himself when things aren't going so well, and that's probably the main thing for me. Yes he is one footed, and yes there may be other issues surrounding him as a Footballer, but all these problems ultimately add up to a lack of confidence which is where the problem lies for me. Sort out the roots of the problem, and the solution will improve.
I think a new start will do him good, and he's not the only player who that can be said about. He has a new chance now - a knew start - and it's up to Valencia, and everyone at the club to do what they can to bring the best out of him.