A 160 lbs fighter would seldom evoke invincibility against the punch of an opponent who weighs 20 pounds more come fight night. It was the last round and Sergio was tired and injured whilst the younger Chavez Jr still had fresh arms since he rarely swung them with power the whole fight. So what if Chavez Jr decides to let his hands go earlier in a rematch? Sergio will then have more to work with and Chavez Jr will get punched out and layed out or look worse at the closing bell.