Or say
@usiel is right and it's all about strength and how it affects his competitive mobility. Foundational deficiencies in his ability to move can compromise his ability to protect himself. And if he has to defend it to the league, he can just flat out say it doesn't feel right, something's off, I'm not jeopardizing my ability to play with my kids, I'm not going out there again until it's better...
Again, there's little precedent for any of this. Those who have tried anything like it have all been cautionary tales. He can honestly say that he wants to play but that it doesn't feel right. Shouldn't take much more than that.
It's like Peyton Manning's injury. No history of a QB having specifically that issue, no history of anyone coming back from it, and if one of the best ever tells you that it doesn't feel right when he throws, who can tell him otherwise or argue that the cause isn't medical?
I really don't think the league would push that hard. Just explain it and if anyone looks skeptical, say "Ow" a couple times.