Willian in CAM provides more defensive stability in our own half, I actually think it would makes tactically better on the counter attack. Cuadrado is a hard working right sided player but isn't in the same league as willian in actually defending. Without the ball, willian could easily slip into a RM/RB when we are defending and defending deep, allowing cuadrado to press in the middle when the space will be wide open (as the majority of the players will be in our half in this situation) which is were he earns his money. Cuadrado hasn't played well enough this season because he isn't an CAM that can play in tight space. Essentially, William can offer a 4-4-1-1 by dropping back and defending like he can, allowing cuadrado and costa to stay higher and attack space. Fabregas will love it. Of course I am particularly thinking of latter stage champions league here, within the league at especially at home I can see Oscar v Willian still being a closely fought battle.