Khedira is the 'defender' as part of the pivot, but he's free moving. Alonso and Schweinsteiger both play the 'creative' role of the pivot, but they are fixated.
So if Ramsey is one half of the midfield, adding a free moving defender like Khedira leaves us vacant in the middle without a fixated midfielder. The idea is that Khedira will be asked to sit, but obviously that would be different to his current role which is where he excels and is the basis upon which we are signing him. So the risk does exist, as it's not his natural role.
I think Khedira and Arteta would form a better midfield, and we should then push Ramsey further forward.
