It's probably fair to say that Arsenal fans have been relatively happy with the tactics Arsene Wenger has employed this season. Barring a few hiccups, the 4-3-3 formation has both improved the defence and given the forwards a lot of freedom to play 'the Arsenal way'.

It doesn't take a tactical genius to work out why the attack has flourished, considering the formation is inherently attacking. With Van Persie playing as a false nine and Bendtner/Eduardo and Arshavin cutting inwards from wide areas, defences had little answer. In the time since RVP's injury, the structure of the play has been more inside-to-out, rather than outside-to-in - although that might say more about the quality of Arshavin's movement off the ball when playing through the centre and the lack of willingness for Walcott to leave the touchline. That's another discussion however.

The brilliance of the 4-3-3 system is that there are fewer players required to do both attacking and defensive jobs. When a 4-4-2 meets a 4-4-2, the only players not effectively cancelled out positionally are the fullbacks. It's no surprise that the best teams in recent years have typically been those whose fullbacks are able to give them extra width going forward. The key to dominating a game positionally in such games has been pushing up the field in wide areas, forcing the opposition's fullbacks and wide midfielders to sit deeper.

The scenario is different with a 4-3-3, mainly because wide forwards are providing most of the width and pinning the opposition fullbacks well inside their own half. As a result, the fullbacks are not required to expose themselves early on, but the formation also gives the side the liberty of being able to play an out-and-out defensive midfielder. Alex Song is the main reason Arsenal have been able to strengthen their defence while playing a more attacking style.

Arsene Wenger said earlier this year he thinks Song may still be a centre back long term. That's not surprising as he's virtually playing there now in any case. The next time Gallas or Vermaelen heads forward, watch as Song drops quietly into the hole they've just vacated. The side have been far less susceptible to the counter-attacking goals - aside from the unavoidable times they've been forced to chase the game - that were the undoing of them last season against sides like Villa and Man City.

The real issue now for Arsenal is how to keep the defence working without the man who has made it work so far this season.

When Song goes to the African Cup of Nations, the worry is that Arsenal don't have a ready-made replacement. The names put forward so far - Ramsey, Diaby, Nasri - would all lend that role a different aspect that would help our attacking play but weaken the defence. In a 4-4-2, one or both of the two central midfielders has to be able to shield the back four, race from box to box and distribute the ball (see Flamini). In a 4-3-3, Wenger has the luxury of being able to play players where best suits their attributes. Song can hold, Denilson can distribute and Fabregas can drive forward and create chances.

There's plenty of midfield cover, but no defenders ready made to step into Song's role (or Gallas's or Vermaelen's for that matter). At the moment, Silvestre has been forced to play left back but is probably thought of as the next best centre-back. I'm not necessarily too enthused about him stepping into Song's shoes.

If given the reins - and it's probably important to note that there's a reason I'm not in charge of a big football club - I'd be giving either Bacary Sagna or Tom Vermaelen the role. Vermaelen would be excellent in that position as a shield in front of the defence, but the negative points to doing this are that there is no one I'd be confident throwing into the hole he's just vacated, and that his passing may not always be pinpoint. With Sagna, you've got a defensively minded player, someone who is comfortable on the ball, although one drawback of this is that his ready made cover – Eboue – will be joining Song in Africa.

Wenger has already proved in Emmanuel Petit that fullbacks can make fine midfielders. In the past decade, Man United have often used John O'Shea or Phil Neville midfield to sure the side up against teams they were worried would pass the ball around them. There's no reason why Sagna couldn't make the same impression for a couple of months when a strong defensive midfielder will be badly needed.