It seems you have never been punched there by a unrully kid, no one recovers that fast, even if you are not in pain you feel unconfortable for a long while
Why do you make it seem like it was something incorrect? The manga showed paisen looking like he recovered, so saying "respect for strength of will brushing that off" isn't wrong. Either your problem is with manga showing him recovering - but maybe kid just didn't actually punch that hard, or it was just a silly manga joke and doesn't need to be super realistic - or if not, then even if paisen was uncomfortable, he did show a strenght of will by pretending to not be affected.
Manga shows character hurt painfully, yet quickly recovering and smiling normally:
"Wow, this guy is tough and strong-willed if he recovered that quickly from such painful hit, respect."
You -> "You must have never be hurt painfully. No one recovers that fast." Like, seriously, what?