There are essentially four variables that determine a player's value: (1) Health; (2) Skills; (3) Team Context; and (4) Role. Of the four, (1) and (4) are most subject to dramatic change, but health is usually both unpredictable and obvious, so let's set that aside for the moment and focus on role.

A player's role determines the quantity of his opportunities (team context determines quality), and quantity of opportunity is a huge factor in value. Once you get beyond the league's elite players, projecting roles becomes difficult, and this is even before we take into account frequent role changes due to other players' injuries. For that reason, it's good to have an idea of who the players are and where they are independent of their immediate roles. To that end, I've ranked the top-20 QB, top-30 RB, top-30 WR and top-20 TE irrespective of roles, i.e., assuming everyone had an equal number of basic opportunities. For QBs, that's 550 passing attempts, for RBs, that's 250 carries, for WRs and TEs, that's 130 targets:

Here's how I'd rank them:

There are some issues with this framework (Would Shane Vereen really get 250 carries even if every other RB on the Patriots dropped dead? And why then did I leave off Darren Sproles?), but for the most part it's a list of players with the most upside, opportunity being equal. That favors running QBs, efficient-passing QBs and QBs with good weapons. RBs who catch passes, are suited to goal-line work and play in good offenses get boosted, as do WRs with good size/speed profiles and good quarterbacks.

It also shows which top players are simply a product of their roles and who are in danger of losing significant value should their opportunities shrink even moderately. Matthew Stafford is a volume guy, as is Larry Fitzgerald, Marshawn Lynch and Jason Witten. Consequently, they're absent from these lists.