I'd like to see him tried there, personally.
On the ball he has a great first touch, is good at passing between the lines, is very good at shielding the ball under pressure, is capable of carrying the ball forwards which is a nice asset to have in central areas (well, anywhere really). He has a nice scoop pass on him, too.
Defensively, he's good at pressing, dealing with 1v1s, positioning himself (though positioning himself as a LB is different to a CM obviously, so who knows if his sense of positioning would translate as well centrally).
With Williams looking good at LB, and us lacking a CM who's more adept as the deepest one, I'd definitely like to see him tried at the base of a 3 behind a McTominay/Fred and Pogba. If he can get fit that is.