Actually, Gadson went on record several times as saying his injury/should/surgery flared up a bunch of times during his career. Also, Tsirtsis(although much more tight lipped about it) was rumored to have similar issues, especially last season.
Lee may very well recover 100% without a notable issue. Still, DI is an incredible grind and areas of your body will hurt all the time when they have never been injured, let alone areas that have been surgically repaired. Plus, all that doesn't factor in your opponents influence. If you know your opponent has a "weak spot", you do your best to work on that, especially when they are as good as Lee. If I were his seriously mismatched opponent, I would be doing my best to work on his shoulder. It probably wouldn't work, but what other options would I have?