Arsenal manager Arsene Wenger insists his side can still qualify for the Champions League last 16 despite sitting pointless at the bottom of Group F after losing to Olympiakos.
A 3-2 home defeat was Arsenal's second straight loss in the group stage, with two matches against German champions Bayern Munich to come next.
"It leaves us in a bad position but we are still in it," said Wenger.
"We have to get a result in our next game against Bayern Munich."
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The Gunners dominated possession against the Greek side but went behind three times as Olympiakos secured their first win in England.
Theo Walcott and Alexis Sanchez cancelled out Felipe Pardo's opener for the visitors and an own goal by David Ospina, who Wenger had selected ahead of first-choice keeper Petr Cech.
But just 59 seconds after Arsenal's second equaliser, Alfred Finnbogason scored the winner.
When asked if it was his worst night in Europe, Wenger replied: "No. Sit on the bench and lose a final in 2006 and ask that question."
What hope for Arsenal?
Tuesday's loss meant Arsenal, who have qualified from the group stage for the past 15 seasons, have lost their first two Champions League matches for the first time in their history in the competition.
The last team to progress after losing their first two group matches were Galatasaray in 2012-13. Only nine sides have done it in the competition's history.
"We have to think we can deal with Dinamo Zagreb and Olympiakos away," said Wenger. "Some aspects of our game were quite good but we lacked quality in our defensive concentration.
"The turning point was at 2-2. We had scored a goal and then straight away we gave them another goal to make it 3-2.
"We had 65% possession but we feel guilty because we gave easy goals away. We had a bit of bad luck as well."
'It's a farce'
Goalkeeper Ospina was preferred to number one Cech and it was his inexplicable fumble that led to the visitors' second goal.
Wenger grew increasingly tetchy during the post-match news conference when asked why he opted to play the Colombia international.
The 27-year-old has yet to feature in the Premier League this season but played in the League Cup win over Spurs.
"Ospina had a fantastic game at Tottenham. No keeper is mistake-free. It could have happened to Petr Cech as well," said Wenger.
"I don't have to give you an explanation about every decision I make. You are capable to judge it and I leave you to do that.
"Petr Cech had a slight injury alert before the Leicester game. I didn't want to take a gamble but it is not because of that we lost the game. It's a farce."
Ospina played 23 games in all competitions last season and kept 10 clean sheets for Arsenal.
"I make the decisions for the team and I'm responsible for it," Wenger added.
"I know many things that maybe you don't know and that you ignore. You can't select a team by making a call before the game and getting everybody's opinion."