Khan definitely has a glass jaw. Being hurt and dropped by light hitters on multiple occasions, then getting flattened by a decent puncher (Prescott hasn't show tremendous power against any other decent foe), meets the prerequisites. Hatton was stopped in two rounds by the best fighter on the planet, Khan was stopped in 54 seconds against a guy who hasn't beaten one other world class opponent, has gone the distance in three of his last four fights, never coming close to a stoppage, and of course he lost his last bout. What happens to Ricky Hatton in the ring is irrelevant to Khan's abilities to take a punch anyways.
Kotelnik is a below average puncher and he never landed one clean shot, so it was impossible to crack that china chin. Credit to Khan for that though, because his defense was top notch.