Arsene Wenger has admitted he is disappointed Arsenal were unable to bring in a defender ahead of this week’s transfer deadline.
Arsenal wanted to sign West Brom’s Jonny Evans but could not get the deal done.
Speaking at a press conference on Friday morning, Gunners boss Wenger said: “Our defensive numbers are not good enough – that’s where we needed some possible strengthening because we have to improve our defensive numbers.
“I believe we have to find an internal solution to our defensive problems and that’s everybody putting more effort in.”
Arsenal did boost their attacking strength with the signings of Henrikh Mkhitaryan and Pierre-Emerick Aubameyang.
And Wenger has suggested he could now switch to a back four in order to make the most of his team’s strengths.
“It’s a possibility. I will adapt to the difficulty we face,” he said.
“I will have to see as well how well the idea works and how efficient it is.
“We are flexible on the organisational front – we can play with a three or a four and that can change game by game.”
Mkhitaryan is set to start Saturday’s game against Everton, but Aubameyang has been struggling with a virus and will be assessed to see if he is fit enough to play.