I'm a Khan fan but his chin is shit.
He's been hurt and decked by blown-up super featherweights with abysmal KO percentages and, as Lance mentioned, before Prescott sparked him he managed to wobble him with a jab, and not a particularly stiff one either.
It's hard to say whether it's improved since he moved up because he's barely been hit flush since Roach changed his style. It's feasible that some of the problem was down to him draining himself to make the lightweight limit, because he's massive for a light welter... how he was making 135 is beyond me. Anyway, Smith-Degale starting now.