The England international centre-back was among the starters as a fine double from Jason Puncheon sandwiched a tap-in from Cardiff old boy Joe Ledley to consign the Welsh club to a comprehensive defeat on Saturday.
Ole Gunnar Solskjaer's charges have won just once in their last nine league matches, and are six points adrift of safety with just five games remaining.
Caulker is at a loss to explain Cardiff's poor showing against Tony Pulis' men at the Cardiff City Stadium, and claims the club's remaining matches are now akin to "cup finals".
"It's difficult to explain it," he said. "I don't know what went wrong.
"It was not for a lack of trying because the boys ran their socks off. But we are going to have to dig deep and show a lot more fight and passion.
"We apologise to the fans that turned up and gave us great support as always. We were very disappointed with it. We know we can perform a lot better than that.
"We've given ourselves a mountain to climb now. It's going to be five cup finals between now and the end of the season.
"It's still mathematically possible we can stay up, so we are not going to write ourselves off.
"We've got confidence from how we played at the start of the season and we've got to get back into some kind of form."
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