He's might be crazy asking Manny for a fight, what a desperate thinking cause he never had a chance to win over Manny or anyone that has been in boxing.
He wasn't thinking of winning in the fight, he was thinking of winning the total money they will generate for the fight, that's the obvious truth. It could be better to fight in Octagon, not in boxing. If Manny will accept the fight, it simply not because of shutting up Conor but also because of money and that was an easy box for Manny either... 
Manny won't accept to fight him in octagon because Manny does not play his sport but Conor has a boxing license so he can fight Manny. Search in google, you can find a lot of article stating that Conor got license in boxing.