The Gunners are in sixth place in the Premier League table and 12 points behind Liverpool FC in the top four.
Arsenal have been hopeless on the road in the 2017-18 season following a poor return of three victories in 15 away games in the Premier League.
The north London side have already conceded 41 times in 30 games in the current top-flight campaign to highlight their weakness at the back.
The Gunners brought in Armenia international Henrikh Mkhitaryan in a swap deal from Manchester United in the January transfer window.
Arsenal signed Pierre-Emerick Aubameyang in a club record £56m deal from Bundesliga giants Borussia Dortmund on transfer deadline day.
Former Chelsea FC midfielder Lampard believes Arsenal have a mentality problem under Arsene Wenger.
“I think it’s a mentality issue,” Lampard told BT Sport.
“I do think that sometimes a bad performance doesn’t get addressed and they have another bad performance. I think that lately they just haven’t been good enough.
“They haven’t recruited to get top-end defenders.”
Arsenal finished in fifth place in the Premier League table last term to miss out on a Champions League spot for the first time under Wenger.
The Gunners have failed to make progress in the current campaign despite spending over £100m on new players in the last two transfer windows.
The north London side brought in Schalke 04 defender Sead Kolasinac in a free transfer last summer.
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