You've seen it a thousand times already. Pac goes insane when someone lands a good shot on him or even tries to pressure and crowd him. Against Diaz, he would be in a non-stop frenzy and it could only end with Diaz getting brutally beaten. What else could he do? He can't hurt Pac, he can't match his power and speed, and he can't suddenly turn into Floyd Mayweather mid-fight. All he can do is keep coming forward, throwing punches, right into the teeth of Manny's buzzsaw combinations. Diaz would get ****ing slaughtered.
It's using hindsight to see that a guy who throws a million punches a round with very little power would get tore the hell up against a guy who throws a million punches a round with massive, knock you the **** out with one shot power? Who's also considerably quicker with his hands? Seriously, I fail to see how this could have been a good fight, unless you like one sided beatdowns.