I don't think he beats Rios. Rios is too relentless, a bad style matchup for Khan. He'll soak up whatever Khan dishes and walk him down and expose that chin. Matthysse is the best at 140; a beast who would knock Khan into retirement. Any of the rough and tumble guys at 140 would trouble him imo.
If he fought smart I think he could beat Rios, he handled Maidana who is a huge puncher at 140lbs and doesn't stop pressuring. I know Marcos isn't as skilled as Rios but I do think Khan could get in and get out before Rios could counter. If he tries to get flashy and trade then he's dead in the water.