Whichever attacking players United sign (Griezmann, Perisic, Bale) they will be made to look average or poor by the dreadful tactics of their manager. Look what he's done to Mkhitaryan, Mata and Ibrahimovic, all once considered world class. The first two in particular are recurring jokes now, nothing more. Mourinho is a bitter, stubborn man who belongs nowhere near a team that call themselves the biggest in the world.
The other big problem is their transfer policy. They buy big names without a second thought to how they will fit in.