I will say he is one of the toughest QBs I have seen. It would not surprise me one bit that he is hurting and trying to not let others know about it. I sure as heck wouldn't want other teams to know I was hurting in certain areas.
There's a difference between "hurting" and being injured. Stafford is NOT injured, as he just said again today, and as he and McVay have said multiple times. However in the NFL "hurting" is an everyday part of the job for almost everyone who plays.
There is no secret injury which caused the 3 game turnover spree. It was a combination of an impatient, aggressive coach, injuries to key players, and a QB who has always had some lapses in attention and execution. Stafford has done this his whole career. He's still an outstanding QB, but he's not Rodgers or Montana in terms of consistency of execution.
I believe Stafford has the potential to be an elite QB. This could happen if both McVay and Stafford realize they need to exercise more patience and better judgment and then commit to it.
A good start would be to give up chasing the illusion that they can "dictate" to any defense at any time by attacking downfield whenever they want. The offense just isn't that good. Neither is Stafford, if he is just turned loose to throw the ball without high level coaching and game planning to maximize his ability and limit his mistakes.
The Jacksonville game was a good example of a more subtle and winning strategy based on patience, limiting turnovers, less predictability, running, play action, and attacking downfield more opportunistically.