An NFL player's opportunity to produce is directly tied to his fantasy value. Oh sure, his skill set will play a definite part in how productive he may be, but without clear opportunity, skill set is useless. For running backs, allow me to suggest a new metric for helping with the evaluation of opportunity: touch attempts. Touch attempts is simply the number of rush attempts plus the number of times that player was targeted by the quarterback with a pass.
Touch attempts alone will give you the pure opportunity each running back had, but it can be dependent on several factors, including game score and weather. If you use this statistic in a vacuum it may cause you to miss trends of how opportunity is being split on a given team, and that's what you really want to focus on as the total number of touch attempts available will fluctuate from week to week. To gauge that split, simply divide the touch attempts of an individual player by the total touch attempts by the running back position on his team.