I don't even want to think about it. I wouldn't say it ruined the Sixers but losing Elton Brand hurt their playoff chances. Injuries are part of the game, but I hate how it effects the outcome.
Boston might have enjoyed a back-to-back championship if KG didn't go down. You could argue that LA won, because of that injury.
It has nothing to do with teamwork, strategy or even conditioning. It can undermine a team's plans suddenly and leave them vulnerable in an area they thought they had under control.
I think the Saints probably would have beaten the Eagles even with McNabb, thanks to the specials teams mistakes. And it actually made the Eagles stronger, since Kolb established that he could play the position. But the Eagles also lost their starting middle linebacker before the first game, and they have been struggling against the run ever since. As a result, they resigned Jerimiah Trotter who has been out of football, in the hope that he could help them stop the run.
Chances of it happening are hard to predict, and it's not something I would wish on anyone. At the very least it ruins the quality of the competition and makes the games less competitive when the best players aren't on the floor.