I mean if it happened to me personally, and I felt even remote pain afterwards I'd definetly want to take legal action. There's no reason it should be treated any differently than any assault assuming the opponent deliberately hit you knowing your guard was down. Not to mention you could almost certainly have a percentage of his purse or other financial compensation, and probably just have the fight postponed... Really not sure how it would be resolved though with that regard. The only scenario I could see where I'd want to just shrug it off and fight on anyways, is if it were against an opponent that I truly hated and had no doubts about my winning the fight. I guess that sort of answers the question though, as all fighters probably have that mentality at the top level.