Yeah, I think there is definitely the mental factor for some players. Learning to trust the surgical repair can be tough. I don't think that has been the case with Diggs. When he first came back last year, he actually was playing decently, but then he got hurt again, either because he rushed back or because the rehab program he followed wasn't good enough (which is what the Cowboys blamed).
This year, I think he tried to rush back again and prove that his rehab was best for him, and he's never allowed himself to completely heal. Now, I don't know what went on with the concussion, but I think the knee has been the problem. And maybe Jerry saying he's not ready to play while Diggs says he is is just another example of Diggs trying to will himself back onto the field (although media said he looked good in the portion of practice they watched).