Arsenal vice-captain Per Mertesacker said the Gunners were well below their best after two late goals secured three Champions League points against Anderlecht in Brussels on Tuesday.
The Londoners had trailed to Andy Najar's header, but goals from Kieran Gibbs and Lukas Podolski turned the game around in its final moments and secured a 2-1 win.
German World Cup-winner Mertesacker said the win was a welcome boost after a run of uncertain performances by the Gunners, who needed another late goal to salvage a Premier League point at home to Hull on Saturday.
"Everyone knows that it was a difficult start for us and our confidence is not that much how we wanted it," PA Sport reported him as saying.
"The character is good, the mentality is good but, football-wise, we lack a few things. Our passing game is not as efficient as last year. We are not at our best. We have to admit that, be honest with ourselves and train harder.''
His comments followed midfielder Jack Wilshere's admission that the win "wasn't the prettiest," and he added: "Personally, it is difficult at the moment.
"It was really a long season last year, and to come back from that World Cup and get the motivation back... I am nearly back, but you can feel there is something missing at the moment.
"I am happy not to have picked up an injury yet and I am confident that I have already been through the dangerous period.
"I am really happy that I am slowly fit and ready for the proper fight during the winter period."
Mertesacker, who has now retired from international football, said resuming Premier League football after an extra break following the World Cup triumph had been "harder mentally."
"I started literally when the season started -- I have never experienced that," he said. "We had already won a cup, the Community Shield, and I wasn't there at that time. I was still on holiday. That was something really strange for us to think about."