"The goal is for my mistake, I believe it is my fault," the Argentine, who has been converted from a midfielder into a centre-back in recent seasons, told reporters at a press conference.
"It is another error, and I've already committed a couple and it hurts. I wanted to clear the ball but it bounced too high. In a match like this one you can't commit these kind of errors.
"I never give excuses and unfortunately the team had to suffer. The result is what it is and it hurts. If we had won, the team wouldn't have suffered."
Barcelona skipper Carlos Puyol, who has recently returned from a long-term injury, could come into the centre of the defence for Saturday's El Clasico against Real Madrid at Camp Nou.