Ditto, mate - since we signed Pogba for a world record fee, it's imperative that we get the best out of him. Unless we transition to a Diamond or even Christmas Tree, spending a comparable world record amount on Griezmann and then hoping that they work things out among themselves is asking for trouble when Pogba plays his best football in a midfield 3 and Griezmann plays his best football as a forward off the shoulder of the 9 - which are kind of mutually exclusive concepts in a 4-3-3 or a 4-2-3-1.
Considering that, Icardi would be great as the prototype #9 in a standard 4-3-3, and then we could buy someone like O Dembélé in 2018 (after the World Cup) to complete the attack in wider areas instead of Griezmann. If we wanted to sign Griezmann with a view to transition into 4-2-3-1, we should've signed proper midfield pivots like Kanté or Koke, not a unique midfielder/attacker hybrid like Pogba who shouldn't be asked to devolve into reductive roles.
This would be much better, but even as a big believer in the manager, I dunno if Mourinho has the balls to to implement it given his reluctance to use it against English teams or in big European games against teams with wingers even at Porto and Internazionale:
That way, Griezmann will be in his best role as the support striker, and we'll keep Pogba in something close to his best LCM/AM role instead of disrupting the fundamentals of his game: