In my opinion, Pac is the one in trouble here. Marquez just has to do what he usually does. Pac definitely has to change something, but he's caught in an awkward position. If he tries to box Marquez and make him lead it seems like he won't be able to pull it off, as seen in some of the rounds in the third fight. Marquez is able to sneak some shots in and then back off again. If he tries to be more aggressive and charge at Marquez like the old Pac then he risks walking into a big counter shot and getting hurt. He's going to struggle either way.
He's also got a chin to withstand anything JMM throws at him, while JMM does not.
I don't think that's a given. Pac has a very good chin, but Marquez has a habit of hitting him when he's off balance and least expecting it, which is why he wobbled Pac in their second fight. Pac's legs actually went for a moment, which is quite a rare sight. Even if you have a great chin you can still be hurt by the punch you don't see coming. Pac doesn't have the time to brace or shell up for all of Marquez's counterpunches, which is why he's so wary when he fights him. He knows that he'll be punished for his mistakes, and while Marquez isn't a big puncher, he does have respectable power that can stun or hurt you.