Consider mcnair and blind at RB & LB albeit so narrow that they act almost as RDM, and LDM. None of them would have to adapt at all and having two defensively orientated but creative capable players MIGHT take the pressure off relying on carrick's passing game.
Now, im not saying darmian cant play at RB, he almost definitely will; but theres a strong possibility that his athletiscm and his ability to play on both flanks can be utilised all at once. For that he would have to start centrally.
Example:
Hazard attacks us on the left, if he beats darmian at RB, then we end up with 3 defenders against their attack because darmian has been taken out of the game.
However, if darmian starts centrally, tracking the attacker to the wide positions ( since he is completely flexible between RB/LB), then even if the attacker surpasses him; we will be left with 4 defenders who are all centrally orientated.
Sorry if its confusing!