the Emirates Stadium, where Greek champions Olympiakos shocked Arsenal 3-2 to leave the Gunners stone last in Group F.
The Greeks, ending a 12-match losing run in England, left pointless Arsenal and their coach Arsene Wenger ashen-faced, and the team’s fans jeering at the final whistle.
After falling behind to Felipe Pardo’s deflected opener, Arsenal equalised through Theo Walcott.
But then a horrendous mistake from goalkeeper David Ospina, selected over regular first choice Petr Cech, allowed a Kostas Fortounis corner to slip through his grasp for an embarrassing own goal.
Alexis Sanchez dragged Arsenal back into it with a second half equaliser, but incredibly the north Londoners imploded again with more slack defending gifting Iceland striker Alfred Finnbogason the winner.
“It is still possible to qualify but it is difficult to swallow losing a game like that,” Wenger said.
“We lost it on a lack of defensive concentration and bad luck. They had four shots on goal and we conceded three goals.
“A goalkeeper (Ospina) can make a mistake. That doesn’t explain that we lose the game. It happened to Petr Cech already this season.”
Beaten in the group curtain raiser in Zagreb Arsenal are now up against it to extricate themselves from the pool stages.
To compound their plight free-shooting Bayern Munich await, the German giants flexing their muscles in Zagreb.
Bayern top the table, Pep Guardiola’s side cruising towards the knockout stages after Lewandowski’s treble, and goals from Douglas Costa and Mario Goetze steamrollered Dinamo Zagreb 5-0.
“We played in a very concentrated fashion from the start and when you take your goal chances, the game was over after 30 minutes,” said Bayern captain Philipp Lahm.
Poland hot-shot Lewandowski has now scored 10 goals in his last three games after netting five in just nine spectacular minutes of Bayern’s 5-1 hammering of Wolfsburg last Tuesday before claiming two more in Saturday’s 3-0 win at Mainz.
“He’s had an unbelievable week behind him,” admitted Lahm. Elsewhere, Zenit St Petersburg remained unbeaten with a 2-1 home win over Belgian outfit Gent to top Group H from Valencia, 1-0 winners at Lyon.