Grealish would help replace Pogba's creativity and quality, however they aren't like for like positionally.
Too many people isolate the player and sometimes how they function, but not how it relates to the broader team.
Example is City: Fernandinho/Rodri, with Silva and KDB or Rodri, Gundogan, and KDB.
The farthest back CM is Fernandinho or Rodri. However, Silva, Gundogan, and KDB are all comfortable defending, pressing, closing down, etc. They have tireless work ethic, just all over the pitch.
City also plays narrow off the ball at the back, when in possession. The inside fullbacks playing as defensive midfielders with their two CBs tucked in behind for a high block. The two CBs have the space behind them and to the channels, the fullbacks have defend the wide areas. If a full back goes up, then a central midfielder like Rodri would rotate towards the vacated space to show for the ball and to account for the vacated space by the fullback.
United don't play this technical and spatial football like City. It would be a massive shift in tactics and player demands/learnings if United were to operate with a potential central midfield trio of a single holding midfielder and two 8s ahead of them in potentially Bruno and Grealish (or Pogba). Can see it working if United played a diamond 442, as they've done a few times.
If United were ruthless and seriously bold, they'd put Grealish in his preferred position of left midfield/left forward, currently occupied by Rashford, and either sell Rashford or try to convert him to full time center forward. Unless Ole or another manager can have a free flowing attacking front line that consists of Grealish, Sancho, and Rashford/Greenwood.