Arsenal manager Arsene Wenger has insisted that a poor decision from the extra officials changed the match after his side were beaten by Olympicaos.
Tomas Rosicky gave the Gunners a first-half lead but two goals in 10 minutes condemned Arsenal to defeat and second place in the group.
Wenger believes that the corner from which Giannis Maniatis equalised was wrongly awarded.
He told ITV Sport: "For their first goal, it was a goal kick, simple as that. Not a corner. What is the fifth referee doing behind the line? Nobody knows. I don't think it was the right decision, but we have to cope with it. After that they scored a good goal.
"It was an interesting game. We had a good first half but dropped physically in the second. Many of the players are not used to that level. We had two or three chances to score a second but after that we know we can be in trouble. We needed the second goal tonight.
"We have to wait for the draw but ideally you would want to finish first."
A much-changed Arsenal side were unable to respond and missed out on a golden opportunity to top Group B after Schalke were held by Montpellier.
The Gunners will now meet a group winner in the last 16 and will find out their opponent when the draw is made on Thursday, December 20.
