David Moyes' side was taken apart by Liverpool as two Steven Gerrard penalties put the visiting side two in front before Luis Suarez sealed the win by slotting home past David de Gea late on.
Rooney, who endured a frustrating afternoon where he saw little of the ball, confessed that the loss at Old Trafford was one of the worst he's suffered during his entire career.
He told MUTV when asked about the mood in the camp after the defeat to their rivals: "It's a nightmare. It's one of the worst days I've ever had in football."
Moyes, meanwhile, insisted that it was difficult to pinpoint where exactly it went wrong for United but bemoaned that it was a tough day for the Premier League champion.
"It's hard to assess the game which was littered with penalty kicks and decisions, some were right and some were wrong," he told Sky Sports.
"It was a tough day for us. At halftime, we were 1-0 down to a penalty kick, but I didn't think there was a lot in it. But to give away one 20 seconds into the second half made it a real uphill climb after that."
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- David Moyes