Thomas Vermaelen has admitted that Arsenal were "awful" against Aston Villa.
The Gunners played their last home game of the season against Villa on Sunday, and the 2-1 defeat at the Emirates Stadium sees them on the brink of slipping to fourth in the standings - should Manchester City win their game in hand.
It was also the north London club's third loss in four games, extending a dismal run that has seen them win only three of their last 14 matches in all competitions.
The Belgian was returning from an Achilles injury to make his first appearance for the Gunners since August - and he believes his team should have been much better.
"I'm happy to be on the pitch again but the game was awful," Vermaelen said, according to The Sun.
"The second half was good because we were the better side but in the first 15 or 20 minutes we were really bad. Our mentality was not good enough.
"I don't think we can accept that as an Arsenal side. We should work on that because, otherwise, it's not good enough."
The former Ajax defender added: "We have to work really hard every day to change that and especially our mentality must be better. We have to change that otherwise we're not going to win a lot of games any more.
"We dropped too many points at home this season and that was key."
Because of their recent failure to pick up points, Arsenal will have to get through an early two-legged qualifying tie to secure their spot in next season's Champions League group stage, if Manchester City manage to win their last two games of the season.
"It should be in our minds that it's a big difference if you have to qualify or not for the Champions League. It's something really important to play for," said Vermaelen.
"If we'd won the last two games we'd be in the Champions League. "But now we have to battle with City for that third spot, so it's going to be difficult."