Are we taking Mikey from the Spence fight? If so, I reckon it'd be similar to the Spence fight: Pacquiao by wide (10-2, 11-1) UD.
I'd even go further considering Pac's punching angles, quickness, and combo variety would prove much harder to block for Garcia which means that he'd take a lot more punishment. I'd say it'd end in a TKO or UD.
The styles matchup would favor Mikey, though — patient counterpuncher. The only question is does Mikey have the smarts of JMM and the balls to walk through fire and brimstone.
Not sure Manny of old would give Mikey all kinds of trouble! It's hard to judge off of the Errol Spence fight Mikey didn't come to win but he would against Manny!