A dejected Wayne Rooney has admitted Manchester United's only hope of salvaging their season lies in the Europa League -€“ a competition they didn't even want to be in.

United's hopes of finishing in the top four took a fresh blow on Saturday as they lost 2-1 to struggling Sunderland to remain fifth in the table, six points adrift of fourth-placed Manchester City, having played a game more.

With the Premier League's top-four sides playing each other on Sunday, United had the chance to make up ground on their rivals but failed miserably.

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As the club turn their attentions to the Europa League on Thursday when they travel to FC Midtjylland for the first leg of their last-32 tie, Rooney confessed they have left themselves a mountain to climb.

"I don't know how it will work out, but it will be difficult to get into the top four now," Rooney said.

Time to stand up, work and take @manutd to the place it deserves

— David De Gea (@D_DeGea) February 13, 2016

"It might be that the Europa League is the way to get into the Champions League. We didn't want to be in the Europa League. But now we'll have to try to win it and start again on Thursday in that competition."

Anthony Martial's deft 39th-minute finish got United back on level terms at Sunderland after Wahbi Khazri's early opener, but they were undone when David De Gea could only help defender Lamine Kone's powerful downward header into his own net.

Rooney conceded afterwards United got exactly what their performance deserved.

"We didn't play well, we weren't aggressive, we didn't win enough second balls, we didn't defend well and didn't create enough chances," Rooney said.

"It's a sad day for us. It was disappointing in terms of losing three points, but somehow we have to try to move on."

The damaging result heaps yet more pressure on United's beleaguered manager Louis van Gaal, who admitted afterward the defeat at the stadium of Light that his best has not been good enough for United this season.

"You cannot close your eyes from the top four being a minimum requirement," Van Gaal said.

One of the few players to play up to his reputation for United this season has been Spanish goalkeeper David de Gea and he took to Twitter after the game to issue a rallying cry to his team-mates.

De Gea tweeted: "Time to stand up, work and take @manutd to the place it deserves."