LONDON -- Arsenal boss Arsene Wenger has admitted he wanted to sign a defender in the January transfer window and that he now has to find "an internal solution to our defensive problems."

Arsenal had targeted West Brom centre-back Jonny Evans and reportedly had a late bid rejected on transfer deadline day.

The Gunners have conceded 34 goals in 25 league games so far -- the same amount as bottom-of-the-table West Brom -- and Wenger said he regretted the failure to strengthen his defence.

"Yes, because our defensive numbers are not good enough and that's where we needed some possible strengthening as well," Wenger told a news conference on Friday.

Arsenal signed striker Pierre-Emerick Aubameyang in a club-record deal on Wednesday and also added playmaker Henrikh Mkhitaryan in a swap with Manchester United for Alexis Sanchez.

But the only defender brought in was 20-year-old Greek prospect Konstantinos Mavropanos, who is not expected to contribute immediately, leading Wenger to admit he "wanted to do more" in the window.

Arsene Wenger has said Arsenal need defensive strengthening. Stuart MacFarlane/Arsenal FC via Getty Images

"I believe we have to find an internal solution to our defensive problems, and that means everyone putting more effort in," Wenger said. "It is not a lack of effort, it is a lack of focus in vital moments of the game.

"Sometimes we do it well, and sometimes not. For example, at Swansea we had 75 percent possession and you always think to attack, attack and you forget to defend when you lose the ball because you are so obsessed with wanting to win the game.

"It is always difficult to find the balance when you have so much of the ball. The league is divided in two now, 25-30 percent possession and 70 percent possession. There are two kinds of leagues -- it is a 70:30 league now. The 70 percent [teams] do not always win, they lose many games against the 30 [teams]."

Arsenal lost 3-1 at Swansea on Tuesday despite leading 1-0, with midfielder Granit Xhaka accused of switching off defensively on the equaliser and goalkeeper Petr Cech then gifting Swansea their second goal with a missed clearance.

Wenger, though, didn't want to pinpoint the holding midfield position as a particular need for Arsenal.

"I don't want to come out on any specific names -- I think it is more of a team problem and a responsibility problem. Take individual responsibility for your defensive role," Wenger said.

The Arsenal boss also said he has no plans to offer to re-sign Alex Song after the defensive midfielder started working out at the club's North London training ground recently. Song, who also played for Barcelona and West Ham after leaving the Gunners, is looking for a new club after being released by Russian side Rubin Kazan.

"The situation with him is that he looks for a football club and I allowed him to come here in the afternoon when the training ground is available, to work on his fitness," Wenger said. "Nothing more than that."