People saying "Would exchange him for Pogba" sound silly, like saying "Will exchange Fred for Mahrez".
Pogba plays most of his games at CM for us, in a midfield 2. We lacked players on the wing(James and Martial injured) and Rashford was going through injury problems, so we tried Pogba on the LW. But Grealish is primarily a LW or a #10. That's where Rashford is our main starter. When we end up buying Sancho, Pogba will be back at CM(Rashford, Sancho, Grenwood, Amad and James as options on the wing).
If we sell Pogba, we need to buy a CM, because we will only have Fred/McT/Matic as CM options. Grealish cant play there. If we end up buying Grealish he will bench Rashford, nothing to do with Pogba. If we are targeting Grealish, his direct competion would be Rashford and Bruno, not Pogba.