This isn't true, though. For example, Moyes played Phil Neville in the centre of midfield, at right back, at left back and even at centre half just to shoehorn him into the side during his first stint. You can argue that that was the most effective use of the squad at the time but it wasn't, Neville was his proxy on the pitch so he played him when others were more suited. Managers definitely have favourites and become blinkered to the alternatives. You see it time and again, managers select players over and over until they are sacked and a new man comes in and that seemingly essential player becomes a bit part player under the new man. If they were essential because of deficiencies in the squad they would still be essential under a new manager, no?