Diego knows Nick's stock is higher than his now and wants to capitalize on it a little, the fight even makes some sense given both of their statuses currently. Thing is, they aren't the same fighters as last time they fought. Diego plays the stand and wang game now, and if he tries that with today's Nick, he will end up with a lasagna face and a L on his record.
Objection, Your Honor. He's reciting the opening to Quantum Leap.