Great NFL players know their most valuable asset to their team is availability. It doesn't matter how talented or gifted a star happens to be, if he can't get on the field then there is very little that he can do to help the team.
This is why injuries are so important in the NFL. Who knows how many more games the Steelers could have won with Troy Polamalu in the defensive backfield? Could the Houston Texans have made a Super Bowl run in 2011-2012 if they hadn't lost Matt Schaub and Andre Johnson?
It's questions like this that can change the course of NFL history, for better or worse.
Unfortunately for several players, their careers are defined by the injuries that prevent them from seeing the field on Sunday. It's a cruel reality for these athletes, who will be haunted for the rest of their lives wondering what could have been.