Nope, no way. Sorry.
Rationally speaking, that sentiment makes no sense because Messi is the best I've ever seen alongwith glimpses of 1996-1998 Ronaldo; and he's probably going to end up as the greatest human to ever kick a football. But in a romantic sense, Messi will always be a 'Barcelona player', and that is not going to change. A bit like say Internazionale signing Giggs at the age of 29 from United. Ryan would've never been one of them because his story as a player is woven into the fabric of United. And similarly, whatever Messi achieves with United will pale in comparison with his Barcelona career - not just in terms of achievements, but the emotions involved with watching a 16 year old breaking their youth records, emerging in the first team, eclipsing Ronaldinho, becoming the greatest of the post-modern era, and then - transcending his era. 50 years from now, Barcelona will be at the forefront when people look back at his career, and (if he joins United) - the new club will only be a footnote - a minor side story.
In a similar vein, Martial is just starting his journey with United. He's already entered United folklore with a few magical moments in his debut season at a young age. And I'd like to see where that goes over the next - however many years he stays at United. There is a chance that he might leave for greener pastures in the future for whatever reasons, but there's also a chance that he becomes 'one of us' - like Messi is for Barcelona, and sticks around for the long haul - adding layers to his United story with each great performance. He might not be an academy player, and a lot of credit for his development goes to Lyon and Les Ulis and Monaco, but he did burst onto popular consciousness at United, and that means something on a deeper level. Martial might not end up being even a quarter of the the player and talent peak Messi was, but I'd still take him for emotional reasons, and the fact that he's young enough to be considered a primarily United great in the future if he sticks around, as opposed to a set-in-stone Barcelona great - who also played in Manchester.