I find that when I change players while defending, it changes to the player outside the ball. So for example, if the ball goes to the left of the field, I hold left direction to get my player running towards the guy with the ball. When I change players, I am usually still holding left and so the guy run away from the ball carrier. Man this is hard explain!!! Do you get what I mean? As they pass left, if the INSIDE player got highlighted, I could continue to hold down left and he would run at the ball carrier... Confused? 
