His position for Koke's goal is textbook.
Alba loses the duel, Mascherano goes to stop the danger (sort of), but he fails, Pique moves as well to try to block the cross, also fails, and Alves takes on Griezmann who was left unmarked. Rakitic should have followed Koke, who was comming from the midfield area. You take action to stop the immediate danger. That's what Alves did. Took action to stop Griezmann, who was attacking the box, to score. Rakitic should have spotted Koke's run earlier, but he didn't. Mascherano is also to blame, even more thank Rakitic maybe, because he does pretty much nothing to stop Saul. His position is the worst I thought.
So, Alba, Mascherano, and Rakitic. Those 3 share the blame.