Arsene Wenger admitted Arsenal were just not convincing enough as they ended their Champions League Group F campaign with a 3-1 defeat in Olympiacos - but the Greeks missed out on the knockout stages.
Despite qualification and top spot already secure before the clash in Athens, Wenger - taking charge of his 200th European tie - expected better as defensive lapses from what was an experienced back four again proved costly as the visitors trailed 2-0 at the break.
"We had the early chances, but Olympiacos played a very committed game and made a good battle," said Wenger. "We lost too many balls in the first half and overall became a bit insecure. I am annoyed because we lost the game, it is difficult to single [anyone] out. Offensively and defensively we were not convincing."
Wenger added: "We could sense there was something more at stake for Olympiacos than us, and that made a difference."
In-form Algeria striker Rafik Djebbour slotted home on 16 minutes after a loose ball was not cut out at the edge of the area, before substitute keeper Vito Mannone - on for the injured Lukasz Fabianski - made a hash of his clearance as David Fuster put the hosts in the driving seat to reach the last 16.
Yossi Benayoun scored a consolation on 57 minutes before Francois Modesto secured victory with a minute left.
However, that was not enough as rivals Marseille produced a late rally to win at Borussia Dortmund and deny the Athens club qualification as they had to make do with the Europa League instead.
Wenger, who revealed full-back Andre Santos was set for a spell on the sidelines after going off with an ankle injury, added: "They were sharper than we were at 2-0, but we came back and then looked like we could equalise, but they scored the third goal. Overall Olympiacos deserved to win.
"Unfortunately for them it is not enough to qualify as Marseille made a super result in Dortmund."
While the extent of Santos' injury is not yet clear, Wenger said the Brazilian would be "out for a while", and also confirmed reserve keeper Fabianski had suffered a "deep cut on the knee".