Breidis Prescott is woeful. Khan would batter him in a rematch. I wouldn't pick him over Casamayor though. David Diaz is a toss-up.
I think that would be a great step-up fight for Khan, but he needs to fight one more durable lighter puncher first. Someone like Ben Tackie might be ideal. Diaz can punch a bit. His KO rate is low, but he knocked out Santa Cruz and fought most of his career at 140.
Id pick quite a few guys in LW to KO Khan, maybe not Diaz. Id pick Soto to KO Khan and even someone like Jorge Solis. Khan's chin is VERY shaky. The 2 times MAB hit him clean with very poor leverage, you could see he really felt it.