It wasn't always a case of one or the other as they played their fair share of games together, plus that isn't 100% true anyway. You make it sound like anytime Skrtel played, Agger was benched.....Agger rarely saw the bench in his career here, mostly injuries that kept him out the side.
Take Rafa's last season for example, Agger started every game of his 36 appearances. Skrtel started 23 of 29.
Under Kenny, Agger started 31 of his 34 appearances, injuries meant he couldn't feature in the others, Skrtel started 42 of his 45.