I would posit it wasn't...
Ignoring that yesterday was probably the most ridiculous day in the history of football transfers, there's been a lot said throughout January that Arsenal needed to sign a centre back. Now, I'm not convinced we needed to sign any players at all - but the wider point is I really don't think our weakest position is centre back...
The starting point for demanding signings in January is presumably that the current squad isn't good enough to win trophies. I'm not sure by that at all. But there are areas where one can say it is true. That Eboué will have to play at right-back in the first leg of the Champions League last 16 game creates a reasonable argument that we should sign a right-back - with Eboué playing, our chances of qualification in the Champions League will be greatly diminished. Is he a nice guy? Yes. But in recent weeks, he's been outplayed by Huddersfield and Ipswich wingers. Pedro will be quaking in his boots as I write this.
The point I'm making is that although Arsene not signing a centre-back is a risk, it could very well pay off - Djourou, Koscielny and Tommy V are all excellent centre backs, and Squillaci isn't nearly as bad as people like to make out. There are few teams in world football who have three centre backs that good.
On the other hand, if Alex Song is injured we have nobody who can really play his position. And on what I've seen this season, if there's one position where we really lack a strong player it's at left-back. Clichy is always knackered after sprinting and ends up out of position; Gibbs is no longer injured but is clearly a man out of form.
I'm quite happy to give them the chance to prove themselves, but a couple of injuries there would hurt us considerably more. In fact, I'd go as far as to say that the desire for a centre back is more based on us historically picking up injuries than anything else. Right now, the partnership of Djourou and Koscielny is pretty watertight.
Arsene decided that he was happy with his lot. Time will tell if it was the right decision, but I think our weakest position is left-back. And even there, we're pretty strong so criticising the lack of signings seems very harsh indeed.
The ultimate judgement from the decision will come from results on the pitch, and as long as we keep winning, even Arsenal fans can't really complain.
On that note, it's a big game tonight against a good Everton side - I'm particularly impressed by Seamus Coleman, and a win tonight would be a good result, and an improvement on last season. Come on you reds!
Keep the faith.