Manchester United striker Wayne Rooney says his side's 3-0 home defeat by Liverpool ranks as one of the lowest moments of his career.
Steven Gerrard scored two penalties and missed a third before Luis Suarez sealed victory for Liverpool, now just four points behind leaders Chelsea.
Manchester United also had defender Nemanja Vidic sent off.
"It's a nightmare." Liverpool-born Rooney told MUTV. "It's one of the worst days I've ever had in football."
He added: "It's hard to take. You have to give Liverpool credit - they played well - but it's difficult to take.
"Nobody wants to lose, especially in this way, in your own stadium. It's not nice."
Rooney, who signed a new contract at Old Trafford last month, had just one chance on goal as Liverpool dominated at Old Trafford.
The Red Devils are seventh in the Premier League table, although Rooney said on signing his new £300,000-a-week deal that he was not concerned by their recent dip.
United boss David Moyes says he was at a loss to explain his side's abject performance against a Liverpool team who moved to within four points of Chelsea with a game in hand.
"It is difficult to explain it," Moyes said. "It wasn't what we expected.
"I thought the players looked in good shape and good fettle before the game.
"We just didn't get to the standards required to beat Liverpool. I have no qualms about the result.
"I have seen confident, well motivated, hard-working and conscientious players in training. I hadn't seen that.
"We are not doing as well as we should be. We are going to have to do better. We are going to have to make ourselves harder to beat and also to create more opportunities.
"I think the job was always going to be hard but if you are asking me is it harder than I thought I would say so, yes."
Asked about United's fading top four hopes, Moyes, who replaced Sir Alex Ferguson as manager last year, admits it will be a tough ask.
"It will be very difficult but it is not over yet," said the Scot.
"So we have to keep working as hard as we can. We have given ourselves a long road."
United's hopes of qualifying for next season's Champions League probably rest on them winning the competition.
To do that United must overturn a 2-0 first leg deficit against Olympiakos in their last-16 tie on Wednesday.
"The players are capable of that," Moyes added. "We have something to go for."
Liverpool manager Brendan Rodgers said he was surprised that Moyes had labelled his side as favourites for the game at Old Trafford before kick-off.
Rodgers said: "I would never say that at Liverpool, even if we were bottom of the league. Anfield is Anfield."
Moyes responded: "I just thought Liverpool were above us in the league and playing well and that any average person would have said the same."