A 2-1 defeat to Swansea City at Old Trafford saw the defending Premier League champions knocked out of the cup at the third-round stage for the second time in five seasons, heaping further pressure on manager David Moyes.
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"We are away to Sunderland and we've got to recover quickly and lift ourselves," he told the club's official website. "That's a massive game now.
"We have to raise our performance level.
"We feel like we've let the manager, fans and everyone down."
United travel to the Stadium of Light of Tuesday for the first leg of the semi-final before the return match in Manchester on January 22.