I voted that it's no excuse, its part of football. I believe that for the most part. You need to groom depth, usually young depth, that can do the job, and be able to scheme to cover where you're left weak.
However, in the case of a couple of teams this year, I lean to injuries being a legit excuse. Carolina is down to some guy who really shouldn't be in the NFL as their starting QB. Some of this is their own fault ( D. Carr ), but to lose 3 QBs will kill just about any team.
The Rams are the other squad that I think has legit injury excuses, losing most of the Oline and QB and RB will slow down most teams.
But look at the best teams. The Colts have played without Harrison on O and Freeney on D, but continue to win games. The Pats started the season with Pro Bowlers Seymour and Harrison on the sidelines and continued to win. I see these teams cover weakness with schemes very successfully, and develop young talent well enough to fit the schemes.