He's 26, and it happens to some players. Torres was busted by the same age just before the 2010 World Cup. It gets to a point where you have to accept he had purple patch for a while and now just isn't as good as you originally thought.
Kaka went the same way at Madrid, struggled in his final season at Milan too. Ronaldinho a good example here too. Whether it be form, personal reason or simply lacking professionalism off the pitch, it happens.
Over the past few matches Barcelona have been pushed more than any other season, their defence is now taking a battering and the unbeatable aura is slipping away - as the defence are being more exposed, so are some of the players abilities.