You know a concussion in the situation they're in makes a lost arm seem like nothing right? Had she gave him a concussion, he would've lost focus of the move before being ripped to shreds by the flying Hollows. And from my knowledge, he has bandages to patch himself up, which means that he'll be able to stop the bleeding. Worst case scenario, Feyora will come to his aid just like she said she would so no need to worry there. Plus since he pretty much destroyed most of them, all that's left is the leader outside of a few stragglers.