The Gunners now face an uphill task to qualify for the knockout stages of the competition with the 3-2 loss to the Greek champions making it two defeats from two, with two games against Bayern Munich facing the side next, and there's pressure on Wenger's men to win at least one of them in order to make it through to the next round.
The game on Tuesday night was remembered most for the costly error from Ospina as he appeared to carry a corner over the line to give Olympiakos a lead at half time. Alexis Sanchez equalised only for the visitors to score again.
Ospina played in the first group game, where Arsenal lost 2-1 to Dinamo Zagreb. It was thought that Cech was being rested for the Premier League clash against Chelsea that followed along with other members of the first team.
The Telegraph's report states that Cech declared himself fit to play after there were worries over a slight calf problem that he picked up in the win over Leicester City, but he looks set to return to the side against Manchester United on Sunday.
From one perspective, one can see where Wenger is coming from - he can't expect the goalkeeper he selected to carry the ball over the line in a game. Ultimately he has put his trust in Ospina in the hope of keeping Cech fresh and fit for a huge clash against the Premier League leaders.
But if Cech has declared himself fit for a must-win game, then your first choice player should be starting. He was seen as the missing piece of the jigsaw by many as he is an experienced and proven goalkeeper and now the selection looks baffling to the watching public.
The only way Wenger can learn from things is to play him in every big game where there is something riding on it, and get a win against Manchester United which will put some of the criticism behind him.