No one said that this was his only weakness, it is one of his biggest weakneses. I dont know if its more or less important, I wouldnt say he is that predictable id say he varies up to the head and body as well as mixing up the punches .. granted he doesnt mix his range too often. I dont think he lunges in much tho ... i cant rememeber him ever really being timed on the way in. He tends to get caught by staying in the pocket too long.
Khan has poor ring IQ not chin. Some of the punches he took against Maidana were monsters. When he is in trouble he tries to fight his way out if it, rather then getting behind a solid guard and getting on his bike.