Don't think Grealish would be well adjusted to play the Pogba role, in the system we play today. You'll notice Grealish to carry the ball and dribble a lot more than dictate play from deep.
In the games I've seen of Aston Villa - I've seen him get a free role popping up across the midfield trying to create something and that's where you'll get the most out of his talents. By buying him, we're more or less saying he'll be a back-up to Rashford or to Bruno and I'm not too sure he'd want to go from playing every match in the PL to back-up especially so early in his career.
I think Grealish is great for buying free-kicks, time, good under pressure, he can carry the ball, he's a complete player for me box-to-box.
I like how he can cross from any angle which is great for wide areas, he won't be drawn central like Bruno is at times, Grealish can play anywhere, honestly Grealish is worth the 50-60M price tag with an add-on of 15-20M in my eyes.
It's not just English hype he's genuinely a top class player.
I don't think he'd be a back-up at all, I think he'd be a player for certain games where he's needed, and he would play in 40+ games a season still, United are in so many competitions that he would still get the same games, and in some games he'd be first pick.
In games where you need width, and you want Sancho, Martial, Rashford in the box, Greenwood can take the wide position.
You can even play a team like... against stubborn teams, obviously Matic would be preferred, but the point is Grealish can offer width and you can have Rashford, Martial, Sancho, Bruno all around the box.
It might even possible to do a 3 at the back and overload the attack, I think United have a very capable squad now, but I do see Grealish as a very good addition.
The only addition post Grealish is a CDM and a CB but that can wait for future windows.