Other than referees, over or underating key injuries in the NFL can cost you a game real quick. Knowing your back-ups and what effect they will have when replacing a starter is an edge you need to use.
When a quarterback goes down from injury you must account for his replacement. Most teams will lose 7-10 points. There are a few teams that could add points such as the Eagles and Buccaneers which have solid experience behind the starter.
The best way to gauge a team that has lost it's quarterback is to compare the margin of victory or defeat to the line in the following game and that difference would be how I grade the power rating. Another self check would be to compare the yards per point, (YPP) to see what the drop off or gain could be.