Mascherano is not to blame for the goal.
He ran back to cover for Pique, who attempted a slide tackle (failed) and left room behind him--which Masche ran to cover. Puyol played the offside without regards to the circumstances and got caught. Masche had to cover Pique's mistake to prevent a through ball through the middle, which is more dangerous than the ensuing through ball to the flank. I blame Puyol more than Masche, but ultimately it was Pique's failed slide tackle that led to the goal. That is why coaches reinforce playing on your feet.
Blame order:
1. Pique: failed a slide tackle, leaving room behind him which was being exploited by run that Masche had to cover.
2. Puyol: caught ball watching and out of position, didn't cover the run behind him.
Anyway, Mascherano was fantastic, as was Pique. Carles was the worst of the three.