I think Ndombele could really flourish here, behind Pogba and Bruno. He was simply phenomenal in Lyon.
Can cover a lot of ground, good physically and technically, good tackler, and a very good passer.
Although he is a DM, he might not be enough like Matic in the way that he's not a pure shield for the defense. But he's a lot quicker, so he can cover more ground, making him better in other situations. His technical ability would also be of great use against the high press.
He might be an upgrade on Fred rather than Matic, maybe? I still think he'd be a very good aquisition, with McTominay as Matic' successor.