Prior to the injury, EJ probably had the mobility and physical skills to be in Brent Burns territory even without Petro's hockey sense. IMO at minimum I think he would have been Byfuglien caliber just on his size/speed/shot combination. Neither of those 2 are as good a defenseman as Petro(especially in terms of actually defending), but I think most of the League would have no problem with that type of player being their main weapon/building block on the Blueline.
The injury unquestionably hurt his mobility and that severely stunted his offensive ability.
I think it's very fair to question just how much the injury affected him and what his upside would have been without it.
I just disagree. Hockey sense is huge for a player to get in that tier, and EJ lacked that. He's never seen the ice like Burns or Petro have.