First of all, this has nothing to do with skill. He isn't refusing the flight because he doesn't have the ability to fly the aircraft without the a/p. Secondly, you can be fatigued without and a/p but not be with one. It takes more energy to fly an aircraft without an a/p. When you say you are too fatigued for that aircraft, you are actually saying I am fine right now, but the extra energy it will take to fly without the a/p will make me fatigued by the time we land.