I'll have to check my recording again, but I thought I saw one pre-game lineup graphic showing both Diaby and Ramsey behind Cesc.
In any case, it seemed there wasn't a good understanding of roles in the first 20 minutes. Ramsey was deep at times, as was Diaby. But neither was holding the role on his own. Once Diaby went off and Song came on, everyone seemed to understand the roles (or trust Song) better and we improved.
This could be players not taking to an assignment (Diaby and/or Ramsey) - could be Wenger not defining the assignment clearly enough (letting two players not accustomed to the role work it out between themselves). All I know is that our shape was better after Song came on.