Do we need Grealish? No. He's a LW/#10 and its among our strongest positions. We need a CM, RB and CF more right now.That however doesnt mean Grealish is not an upgrade on Rashford. For eg : Just because we have Maguire and Varane and dont need a CB, doesnt mean that Ruben Dias isnt an upgrade on both of them. He is better(not by much), but we dont need him.
For me on current ability, Grealish is an upgrade on Rashford/Sancho for the LW position. However I think Sancho has a higher ceiling. And for me, he is as good as Bruno(although different type of players). If you dont count penalty goals, there's not much between them
statistically. Non-penalty G+A/90 for Bruno is 0.61, while for Grealish is 0.66. Grealish has better keypasses per 90, his passing accuracy is higher, and is a better dribbler. And lets not forget he is playing for Aston Villa. I would personally not swap Bruno for Grealish though, because although I think football wise they are equals, Bruno is our talisman and leader and that cannot be quantified with stats.
But yes, we dont need him. Lets see how he does at City. I think he will prove a lot of people wrong, those who are saying he wouldnt get in this United side, or that he's no better than other City players.