Because we changed our tactics. We do not need a centre back going into midfield. We now play Gvardiol left wing and Foden is now the player moving from their starting position into midfield. With Gvardiol playing higher, Ake has been getting the nod because he’s left footed so gives us the best balance in the back 3 that remains once Gvardiol goes to the wing. With Ake injured, Akanji will now do that role. So it’s purely tactical. This set up has been working and Stones doesn’t really fit in it.