This post may be a bit late, but its pertinence remains valid, since we will be analysing consolidated (defensive) processes. Tito Vilanova was appointed as Pep Guardiola's successor in order to maintain a certain philosophy, one that has proved fruitful over the past few years. While it's true that most of the major guidelines subsist, it's also true that Vilanova has virtually given up on the three-man defence (except when his team are trailing) and that the defensive pressure is not as intense and effective. Let's take a quick look at the match versus Real Madrid from October 7.
Real Madrid goal #1
Real Madrid were being successful in shifting the ball from side to side, namely through Xabi Alonso's accurate long balls. The ball had just traveled from left to right, with Özil moving it back to the centre. With a far less intense defensive pressure, Barcelona's wingers seem unsure of what to do without the ball and often forget about defending. In this particular case, there are five Real Madrid players in the box against an equal number of defenders. Notice how Ronaldo (blue) is left one on one against Dani Alves with a huge space for his trickery (shaded area).
When the ball gets to Benzema, one of the centre-backs comes out to meet him, as he is supposed to, and Dani Alves (orange) hesitates between covering for this team-mate or worrying about Ronaldo. Again, the right winger is nowhere to be found and neither Xavi or Fàbregas (the midfielders in this particular match) help out near the box.
That simple hesitation is enough for Ronaldo to get past Dani Alves (orange) and bury the ball in the net with a powerful left-footed shot.
Real Madrid goal #2
For Real Madrid's second goal, there was another pressing issue. Barcelona have been less precise and giving the ball away more cheaply recently, especially in tougher matches. Here Barcelona have once again lost the ball and the team take longer than expected to get back into (defensive) shape. The middle is completely unprotected (shaded area) and Dani Alves is not between the goal and his man, as he should. Furthermore, there is no one goalside of Özil (inside the shaded area).
With none of the defenders coming out to meet Özil, Ronaldo sprints past Barcelona right-back. With no pressure from midfield (the two shuttlers are very far from the action), the German international can pick his pass and the offside trap is no obstacle for Ronaldo's speed and intelligent movement.
Despite their perfect start to the season up to that moment, Barcelona's displays have been far from perfect. Their possession has been sloppier at times, but most importantly, their defensive approach seems to be changing, voluntarily or not. The immediate pressure they used to exert after giving the ball away is nowhere near what it used to be, but their defensive approach and positioning have not changed accordingly, which partly explains the sudden hike in conceded goals.