Manchester United were shunted towards an unscheduled exit in the Champions League after crashing to a shock 2-0 defeat at Olympiakos in their last 16 first-leg match on Tuesday.

Goals from Alejandro Dominguez and Arsenal loanee Joel Campbell gave Olympiakos victory at an ecstatic Karaiskakis Stadium, to leave the Greek champions on the brink of the last eight for the first time since 1999.

It was the first time United had ever lost to a Greek team, but their error-prone display was fully in keeping with a season in which the self-belief of the Alex Ferguson era has thoroughly abandoned them.

Previously unbeaten in the group phase, David Moyes's side conceded defeat with a whimper on a chastening night in the Athenian port of Piraeus, failing to muster a single shot on target.

Eleven points off fourth-place Liverpool in the Premier League, their hopes of playing in next season's Champions League were already fading, and they now face a fight to stay alive in this season's competition in the return leg on March 19.

"It's the worst we've played in Europe, that's for sure," said Moyes.

"It was a really poor performance. We never really got going from the start and we didn't deserve anything because of the way we played.

"I take responsibility. It's my team. We didn't play well tonight and we have to play better, but we know that we can do.

"The one good thing is that there's still a second game to come."

Olympiakos coach Michel said: "Today's match shows we've done a very good job. I'm obviously very happy with this match, but most of all, I'm proud of my players.

"But if somebody says that 2-0 is enough to qualify, he's probably wrong. We have a 2-0 lead against Manchester United, which means nothing."

Michel had wondered in his pre-match press conference whether United's players would feel "burdened" by expectation, and in the opening stages his side were happy to let the visitors make problems for themselves.

- Sloppy start -

A sloppy pass from Rio Ferdinand gave Hernan Perez a free run down the left flank and although that came to nought, moments later Nemanja Vidic had to launch himself into a last-ditch block after an untimely slip by Chris Smalling enabled Dominguez to drive into the United box.

Perez then slammed a shot into the side-netting after leaving Smalling in his wake, and with a wayward Tom Cleverley shot the best United could muster in reply, the hosts' 38th-minute opener came as little surprise.

Campbell's cross from the right was only partially cleared and although Giannis Maniatis's low shot appeared to be heading straight at United goalkeeper David de Gea, Dominguez's opportunistic flick sent the ball skidding into the opposite corner.

It was the first time United had gone behind in this season's competition, and there was worse to come.

Kostas Manolas headed a Wayne Rooney free-kick narrowly over his own bar before half-time, but Olympiakos were the first side to threaten in the second half, with Perez working De Gea from 20 yards.

Despite United's need, the English champions were unable to establish a foothold in the opposition's half and in the 55th minute they found themselves 2-0 down.

It was a goal that shone with the invention that United had been missing, as Campbell picked up the ball on the right, insouciantly nutmegged Michael Carrick, and arced a 25-yard shot into the bottom-left corner with his left foot.

The goal drew a disbelieving roar from the home support and within moments Moyes had reacted by introducing Shinji Kagawa and Danny Welbeck in place of Cleverley and Antonio Valencia.

But still Olympiakos came, with Michael Olaitan lashing a snapshot narrowly over, and when a presentable chance finally fell to United's Robin van Persie in the 82nd minute, he hoisted a wasteful shot into the stands.