It was a disgraceful referee... You don't stop the fight 35 seconds to the end, and with 1 fighter able to defend himself. You don't allow a fighter continue with a broken jaw. If you stop the fight is the moment the other guy says its jaw its broken, not wait until the last moment and try to save your face in stopping in the last moment. Bad referee, if this is the best they have in the business...
The moment Usman won the fight, was the moment they told Covington has a broken jaw (which was his mistake saying (he probably have quit thoughts on that moment he said to his corner, or he wasn't thinking too much), but on that moment they told Usman that he had broken jaw (which surprised massively Usman, because it was a round clearly won by Covington, but that give him too much moral to be stopped). If you know the other guy is complaining about broken bones, means he's thinking of quitting, so that gave him a great moral bust, and a focus area where to inflict the damage necessarily to win.