One of the more unique things about professional football in the salary cap era is that nearly every team has at least one position that is significantly weaker than many of the others. While it varies year-to-year, the position of biggest need is usually the focus of said team as they head into free agency and the draft. For the Pittsburgh Steelers, running back is not going to be high on the need list but it is an area that lacks any form of certainty compared to other positions.
The position is unstable because from the top down there is a curious amount of uncertainty.