Football Outsiders devised a stat called DYAR, Defensive Yards Above Replacement, which factors in the defenses a QB plays, his supporting talent, and emphasizes yards in high pressure situations (tight games, 3rd down situations) over yards piled up during garbage time or when the game is lopsided. The DYAR stat shows how much true value a particular QB has to his team.
Among NFL QBs with a minimum of 100 passes attempted this season, Oakland's JaMarcus Russell is the worst with -730 DYAR. That means a replacement-level QB could have thrown for 730 more yards in Oakland's offense than JaMarcus Bustell. KC's Matt Cassel is 2nd worst with -555 while Detroit rookie Matthew Stafford is 3rd worst with -544 (but at least he can make the excuse that he's a rookie still learning on the job).