Grealish is mobile, works hard and has played as an 8 before. For a lot of games we play I think you can get away with Bruno and Grealish as 8.5s in front of a mobile DM. Grealish also offers backup to Bruno as a 10 if we play that system or also on the left...a bit like Pogba who is no longer trusted in the pivot unless we are against average/poor sides and he has Matic beside him.
That would mean changing system and Ole is not moving away from his beloved 4231.
You only have to look at the midfielders he's targeting (Caicedo) to know where Ole wants to go and what we have seen so far this and last season. Two dynamic midfielders, thus allowing our front four and full backs (could improve on this) to cause havoc, which more often than not, they do.
Ideally, what we need is a deep-lying playmaker in the mould of Pogba to play next to Fred/McTominay.
If Grealish comes to United, I can only imagine it'll be to be our left winger which will mean moving Rashford to the right. However, it would make more sense to get Sancho as he plays on the right, meaning Rashford can stick to his favoured left side.