It's got nothing to do with how attacking he is. I think its more to the fact of how versatile and positionally aware he is already.
Except he really isnt that aware for the role he will play deeper for Chelsea, as Alcaraz and I discussed. He has been very lacking in defensive concentration and defensive positioning, mainly since he hasnt had to do it at club level, but even as AM you need to show defensive awareness to the threat, (Rooney for United is a very good example of this) this is a weakness of his.
And again Lucas and Ramires, two very positionally aware players, especially Lucas, both still found it tough adapting.