Manchester United's Wayne Rooney (L) reacts during their Champions League round of 16 first leg soccer match against Olympiakos at Karaiskaki stadium in Piraeus, near Athens, February 25, 2014. [Photo/Agencies]

ATHENS - Olympiakos Piraeus made it fifth time lucky on Tuesday, sweeping past an abject Manchester United team 2-0 in their Champions League last-16 first leg at the Georgios Karaiskakis Stadium.

Alejandro Dominguez opened the scoring with a clever first-half flick and Arsenal loanee Joel Campbell added a sublime second goal to leave United facing a huge task to overturn the deficit in the return match at Old Trafford next month.

"That's the worst we've played in Europe. We didn't deserve to get anything," United manager David Moyes was quoted as saying by the BBC. "I'm surprised, I didn't see that level of performance coming.

"I take responsibility, we have to play better. The players are hurting."

Defeat represented another body blow in an inauspicious season for the English Premier League title winners.

The Champions League is United's last chance of a trophy and they looked sluggish and disjointed against an inexperienced Olympiakos side who are bidding to reach the quarter-finals for only the second time in their history.

It was the first time the Greeks had beaten United in five attempts.

Moyes opted to start with two wingers, Antonio Valencia and Ashley Young, but neither made an impact in a first half in which Olympiakos displayed real energy and hunger.

After a cautious opening from both sides amid ear-splitting noise from an enthusiastic home crowd, the first chance fell to Olympiakos with Dominquez bursting through the middle on a solo run only to be denied by a last-ditch tackle from Nemanja Vidic.

Midfielder Tom Cleverley registered United's only effort of note in the first half with a 17th-minute half-volley that sailed high over the bar.