The offensive staff(s), schemes, and overall offense that JF was in charge of can simply be described as inept. I know that you point out QB injuries, OL injuries, and QB meltdowns, but regardless of any of that, THIS past season it was 100% JF's offense and remained healthy and the offense was actually worse than the previous years offense.
To finish dead last in offense in back to back years and to have consistently fielded an offense that ranked near the bottom of the league for most all of his tenure, well yeah an young, offensive minded coach brings things to the table that JF simply didn't.
We'll certainly find out more when the rubber meets the road this upcoming season, but I'm pretty certain if nothing else, the Rams won't be dead last in offense and probably won't be bottom 5 regardless of injuries or any other factors.
