I'm thinking that auba's best position might on the left in a team that has zero creativity in the midfield. If we had a proper #10 or ozil in the mid than I think auba continues his form from last year up top. With him up top and no creativity or service, not even coming from the wings, he needs to be on the left.
He gets to touch the ball more again and can go back to creating space for not only laca but for himself and allowing him to run in behind. I noticed after his switch to the top, that he barely touches the ball or has the lanes to run into unlike the previous times on the left.