Needs to grow, fill out, and not get bullied. Seems to have the feet for the job, but at the top level its whether he has the physique and the head for it. As a full back in this line up, he not only has to play against the opposite winger, but he has to be a right midfielder going forward and big and strong enough to challenge the opposing centre forward at the back post. When you watch people like Zlatan pull off their centre half onto the back post, it is hard to see Maffeo being able to do anything with that. Fair do's though, as Zaba and Clichy have also struggled with that over the years.
With a little maturation and time in the weight room, he MIGHT save us £20M. Same with Angelino, who is a step ahead of him at this point. Tosin is ahead of both of them, though.