losing pogba would hurt massively, especially in terms of marketing, but tactically we could cope without him. especially if we decide to switch to 4-2-3-1.
back four
Khedira/lemina-Marchisio/asamoah
cuadrado/berardi-dybala--new winger
-----------morata/madzukic--------
losing morata would suck. mainly because of how important he is in big games. but we could also cope without him if mandzukic and dybala gel in. a lot will depend on them to lessen the impact of his loss. ideally though he would stay
Moving from systems with 3-man midfields to 4231 would be expensive both in terms of learning curve and the transfer market. I don't see it happening. I'm pretty sure Allegri will stick with 433 or Zona Mista as his primary formation. Probably 433 because he won't be lacking a RB like he is now.
We can switch between 433, 352, 4312, and a Zona Mista with 3 in midfield relatively easily because all of them are dynamically the same in the middle with a 3-man midfield. Moving to 4231 would be a bigger change that'd require more serious tactical shuffling. And it wouldn't be easy for us to switch back to 352 or 4312 in certain games. Also, Dybala is best as a SS, not a treq.
We already need to sign Cuadrado, Berardi, and probably a LWF/LAM on top of that. Switching to 4231 would mean signing yet another attacking player on top of that which probably isn't feasible.
Pogba is our best player and one of the best CMs in the world when in form, but we may be able to replace him with another decent mezzala. The new CM just won't be as good as an in-form Polpo, won't have Ballon D'or winning potential like Pogba does, and just not being able to see Pogba in our colors would hurt, but we could do a decent job of replacing him, and the $ Pogba's sale would generate would enhance quality in other positions as well.
Good CFs are hard to find, and quality in that position is even more important, so Morata's sale would hit us harder. Mandzukic is a good Llorente replacement. He simply isn't good enough to lead our attack.
More chance of Pogba leaving than Morata though.