Well Arsene is being rather contradictory in his statements on this matter. On one hand he says that the squad needs one more midfielder, preferably a defensive minded one. On the other hand he is saying that he is happy with his squad even if he doesn't add anyone else, and he also said that we aren't close to signing anyone.
My biggest fear is the African Cup that is coming up next January, Song is the only ball winner we have in midfield, and with him gone for 3 weeks plus, i don't know how we're going to fare.
Ideally we should bring in a central midfielder that has ball winning capabilities. But there really aren't plenty of those in the market.