He's one of the top 10 key passers in Europe if you're measuring it per 90 minutes. Nobody above him is available or would be any cheaper.
But it's clear from the positions he drifts into that he's a natural central player and not a winger. His crossing is a bit shit compared to the likes of De Bruyne. If you focus on his key passes through the middle, it actually places him even higher on the list.
And given how poor United have been at beating the press, it would be invaluable to have a player with dribbling ability and a tight touch. His ability to win fouls could be crucial in difficult games. It's definitely a weakness for the team right now. Grealish is the most fouled player in Europe.
There are definitely things he's better at than Pogba or Bruno. And the gap in quality isn't that big to start of with.
He'd get plenty of games, and wouldn't be considered a bench player by default. The three of them would be rotated and/or selected according to the opponent's tactics.