Arsene Wenger says Arsenal are "defensively naive" after their 2-1 defeat by Manchester United.
Arsenal were caught on the break as they pushed to equalise after Kieran Gibbs' own goal and Wayne Rooney doubled the visitors' advantage.
Olivier Giroud's late goal counted for little and the eighth-placed Gunners have endured their worst start to a campaign in 32 years.
Wenger said: "At the moment defensively we are a bit naive."
Arsenal had 23 shots on goal, nine of which were on target, with Jack Wilshere and Danny Welbeck - facing his former club for the first time - guilty of missing chances.
Only Giroud's searing strike in stoppage time was good enough to beat United goalkeeper David De Gea, who was named man of the match.
Wenger said: "It's a game that we dominated for 80 per cent of the time and we haven't dominated a game like that against Manchester United for a long time.
"At the end of the day we were not efficient enough in the final third, or defensively, and we made a mistake at the back which they took advantage of.
"Once we were 1-0 down we had possession in the final third that they had to defend. We missed a few opportunities in the second half, even in the final 20 minutes."