The Arsenal Wenger’s football manager paid the price for an error scattered display that condemned them to a second successive Group F defeat that could ruin their hopes of qualifying for the last 16.
After falling behind to Felipe Pardo’s deflected opener, Arsenal equalised through Theo Walcott.
But Wenger’s surprised decision to select Ospina instead of regular first choice Peter Cech failed in spectacular fashion late in the first half when the Colombian allowed a Kostas Fortounis corner to slip through his grasp for an embarrassing own goal.
Alexis Sanchez moved Arsenal back into it with a second half equaliser, but incredibly the north Londoners fell down again with more slack defending gifted Iceland striker Alfred Finnbogason the winner.
Beaten on all their previous12 visits to England, Olympiacos had caused one of the great European upsets and Arsenal, who face group leaders Bayern Munich in their next two Champions League fixtures are in severe danger of being knocked out from the tournament by early November.
After losing their opening group game at Dinamo Zagreb, Wenger acknowledged there was no more margin for error.
Yet he still made five changes from Saturday’s win over Leicester, including the gamble on Ospina that will prove so costly.
Yet Arsenal’s relief was short-lived, with Olympiacos regaining the lead in comical fashion in the 40th minute.
There shouldn’t have been any cause for alarm when a Fortounis corner drifted harmlessly towards Ospina and without much pressure, the keeper let it drop from his hands as though he was handling a grenade and the extra official behind the goal ruled it had gone over the line before he could recover it.
Worrying, Wenger’s team contrived to shoot themselves in the foot yet again just 59 seconds later.
Esteban Cambiasso clipped an astute pass over the Arsenal defence to the unmarked Pardo and he drilled a low cross to substitute Finnbogason, who eased in front of the flat-footed Mertesacker to flick his shot past Ospina from close-range.
Roberto produced a fine save to keep out Mesut Ozil’s free-kick and Sanchez headed narrowly wide, but it wasn’t long before Arsenal’s shell-shocked players were trudged off with the sound of jeers ringing in their ears.
