The Gunners missed gilt-edged chances in their 2-1 home loss to Chelsea yesterday to fall seven points behind the league leaders and the Frenchman wants things to improve if they are to be a genuine title contender this season.
"We can be much better. Technically we were average, and also so was our authority at the back," he said.
"When you play at home and concede two goals from set-pieces, it is unacceptable.
"I think there is some psychological damage because we lost the game and the players are very disappointed, especially because we have lost a game we should not have lost and that is where we have some work to do, especially in some aspects of our defensive game."