Not sure I agree.
Grealish is not 0 defensively, actually he has a pretty decent workrate. He has played multiple times as the advanced #8 before, and that's his best position, he says so himself as well. Even when he played at Villa for the left, if you look at his heatmaps, those are the positions he occupied, kinda like Pogba. Morever, City play with their fullbacks tucked in helping out their DM, so they are fine with two attacking #8s anyway. Grealish rarely loses the ball and is press resistant, a very good fit in that position for City.
Also disagreed that City need someone like Rashford. City's wingers need to hug the touchline, and stay very wide. Rashford plays best closer to goal from the left, and when he has space to run into. He would be a horrible fit at City.