He's an extremely talented fighter but unfortunately has a very real and serious problem with taking punching. Not so much as it knocking him out but it completely destroys his equilibrium. Never before have I seen a top rated fighter with such a problem losing his legs when getting hit with a decent punch.
Watch Khan vs Scotlands Willie Limond , Limond was a soft puncher and even he floored Khan , further more if you watch the fight Khan should have been stopped , as he dropped again on the count http://www.youtube.com/watch?v=UG1cBjZpFMQ