PIRAEUS, Greece (AP) -- Just when Manchester United thought its troubled season couldn't get any worse, it did.
The English power slumped to a 2-0 loss at Olympiakos in its second-round Champions League opener on Tuesday night, leaving manager David Moyes on the brink of a trophyless first season at Old Trafford.
"It's the worst performance in Europe, that's for sure. It was a very poor performance, and we didn't deserve to get anything. To a man there was nothing," said Moyes, who took last summer over following Alex Ferguson's retirement. "I take responsibility. It's my team and we didn't play well. There is a second game -- we'll do everything we possibly can."
Alejandro Dominguez deflected Yiannis Maniatis's shot past David de Gea in the 38th minute, and Joel Campbell caught the goalkeeper off guard with a well-placed strike in the 55th minute at Karaiskaki Stadium.
"This was a great victory against a massive club like Manchester. We were told to hold the midfield, and we did," Maniatis said.
United is just sixth in the Premier League at 13-8-6 and trails Liverpool by 11 points for fourth place, England's final berth in next season's Champions League. Winning this year's competition might be the only route into next year's tournament for United, which would face a major revenue loss if it fails to qualify for Europe's top club tournament.
"We just haven't been getting the right results," United midfielder Michael Carrick said. "As players we have to stand up. ... That's the business we're in."
The second leg of the total-goals series is in Manchester on March 19.
"I will never say anything bad about Manchester United, because I have such great respect for them. The players know that we still have more to do," said Olympiakos coach Michel Gonzalez, whose team has a 20-point lead in the Greek Super League. "But I can't hide it. I am very happy. It's the players I'm so proud of."
United had the 55 percent possession but was outshot 12-5. Wayne Rooney failed to threaten and Robin van Persie missed his team's best chance, skying a shot over the crossbar following Chris Smalling's cross in the 82nd minute. Olimpiakos became the first home team to win in six second-round matches.
In the night's other game, Robert Lewandowski scored twice and led Borussia Dortmund to a 4-2 victory at Zenit St. Petersburg.
Dortmund, which lost to Bayern Munich in last year's final, took a 2-0 lead when Henrikh Mkhitaryan scored in the fourth minute and Marco Reus added a goal 70 seconds later.
Zenit, playing its first competitive game since Dec. 11, cut the gap on Oleg Shatov's goal in the 57th, but Lewandowski made it 3-1 four minutes later. Hulk converted a penalty kick for Zenit in the 69th and Lukasz Piszczek scored Dortmund's final goal two minutes later.