This content is protected Look at how many times he got hit flush on the chin with solid punches here. If he had such sparring wars on regular basis then it's no wonder his chin is total glass now.
Yeah, that should've been a KO loss. It's all relative but I think Khan's personal life of frat boy partying lends to his chin as well. He has absolutely no discipline at all. I'm sure he does things like coke as well.
I'm not saying he didn't have a weak chin already but I think these sparring wars ruined it even further.