Not sure "sacrifice" is the right word though. That makes it seem like that player is essentially worthless, other than serving Pogba. Or, put another way, that you are basically having to field two players to do the job of one - you are essentially playing with 10 men.
Maybe Im being a bit pedantic here but I think you can look at this differently.
Maybe a certain player - Player A - needs another specific type of player - Player B - to play to his full potential. But then Player B may also excel in his specific function if he is next to Player A. Player A hates defending, but Player B cant pass for shit. But if they play together then can be complementary, they can have a kind of symbiotic relationship, covering each other's deficiencies. They can, perhaps, be more than the sum total of their parts. So rather than seeing it as a form of sacrifice, where Player A + Player B = 1 functioning midfielder, it is actually A+B = more than 2 players.
I dont know if this makes any sense at all outside my own head.
But if you are still with me, if you need B to complement A, or visa versa, and A or B is less than 1 on their own, but more than 2 together, what you are really sacrificing is flexibility. That doesnt have to be cripplingly problematic as long as we have backup options. If Pogba needs Carrick, its OK as long as we also have Blind who can do the job nearly as well. If he needs Herrera... well, then we need someone else who can fill in for Herrera. Schneiderlin maybe.
It would all certainly be much simpler if Pogba could just take the game by the scruff of the neck, and was rounded enough to do whatever was needed. I must admit I kind of assumed for the money we were paying that would be what we were getting. It is a worry, but its still early days and I think he'll develop that ability. I trust Mourinho knew what he was buying and has a plan.