Erikson plays in the same position, is at least as good at the things isco is good at in midfield and also currently has the second highest assist tally in europe. To a large extent it is about assists and goals, we don't create enough chances or score enough goals to be a genuine top side. Eriksen would help in that respect, isco wouldn't greatly. Pogba at present is our only consistent creative player so if teams manage to nullify him then we really labour along. With two similarly creative players in the middle of the park it would be far more difficult for teams to close off the danger. Look at city, they've got so many creative options in and around the midfield. It's very difficult to stop them all.