It would be a very close fight with Rua probably winning in the end, he'd simply be more active and land more overall. Mousasi would box him up bad and try to give as much as he'd take, but Rua's chin was so good back then nothing would stop him, he'd simply continue kicks, knees in the clinch and go for takedowns, it would also be a BJJ/GnP war on the ground. Ultimately, the difference would be Rua having more control because of his underrated takedowns.