You can't worry about injuries. Every team has them and the good teams have capable players who can step in and perform. You have to be able to replace injured starters throughout the year. Last years Packers are a great example. They overcame numerous season ending injuries to key players and still won the SuperBowl.

For most teams, a franchise level QB (i.e. the Rodgers, Brady, Brees, etc) is the one player you can't loose and would be nearly impossible to replace. (See the 2011 Indianapolis Colts) While the Steelers did win 3 out of 4 last year without Roethlisberger, that was at the beginning of the season where the play is not as intense as it is in December/Januaury. It would be tough now to make a Super Bowl run without Ben under center. As long as he can play and perform at a relatively high level down the stretch and into the postseason, the Steelers will be a tough out.