I feel like pinpointing the ONE problem with the offense is like getting mad at algebra for not being as simple as addition.
I don't see a linear equation like Bad O-line Play + Bevell Playcalling = Crap Offense. Certainly, the eyeball test shows some big problems (injury, inexperience, unexciting play calls, etc) which create other little problems (out of position, bad footwork, bonehead mistakes, trying to do too much, relying too heavily on improvisation, etc). But there are too many variables (incuding defensive schemes and execution) to isolate one as the broken part.
Percy will be an altogether new variable, one that may or may not fix the equation, but will undoubtedly change it. Based on his past production, it's reasonable to assume he can help us change the balance of the equation in our favor.
Having said that, I can't find fault with Kelly's critique of Bevell. I just don't think making those changes is the cure-all. Neither is adding Percy. Our Seahawks are going to have to add, subtract, multiply, and divide, pull some hoodoo voodoo to the Nth power before the offense really starts to click. Oh, and get our players healthy and on the field too.
Bottom line, with the intelligence, work ethic, and athleticism of our Seahawks from top to bottom, I have faith that the offense will continue to address mistakes, continue to develop a more complex attack, and become the cohesive unit we all seem to feel is possible.