I seem to have been playing this game for a lifetime and I still have no idea how the injury thing works. I've maxed out in Med/Trng and still had a number of 95+ health guy go out for the season and conversely had some with health around 50- stay injury free season after season. I just expect that every year a number of my guys will go on the DL. To me it's just a HBD thing that I appear to have little or no control over and I've learnt to live with it.