Luis Suarez scored four goals and Lionel Messi netted a hat trick as Barca gained the advantage in the first leg of their Copa del Rey semifinal.
Los Che are winless in La Liga under Gary Neville and the coach has refused to resign in the wake of the trouncing, despite calls for him to apologize for and bring an end to his disastrous reign to date.
Mustafi was sent off just before halftime with Barca already leading 3-0 and the Germany international refrained from blaming himself for the final, humiliating scoreline, insisting that everyone involved needs to atone for what was a shocking display.
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"We want to say sorry to the supporters and to the people who are working for the club," he told reporters. "But you can't ask for forgiveness without giving a reaction. It's not the time to talk about other sides.
"We have to lift ourselves up and that is not easy. In the coming days we have to front everything like men. I do not look at who made the errors. In the good and bad moments we are together as one. The dressing room is in the s**t at the moment, but I believe in this team, and that is why I signed for this club. We were a team last year, and we are this year.
"For many people this has been the worst game of their career. We need to lift our heads like men and grow. The dressing room has to lift. The return leg is not to overcome the score, but to ask for forgiveness."