Their formation also isn't going him any favors either. Although, maybe that goes back to "why acquire this player then?"
To an extent, but where do you play him? Centrally? He basically would leave the center backs with a free play out of the back. He also doesn't come back to harry the midfielders either. On the wing? They've tried a bit, but he doesn't make any pressing moves, so, AWB has had to basically double time, or in some cases the midfield has had to fill in the pressing gap. It's also taken away what seemed to work for midfield, in allowing Pogba a bit more space out of the middle.
As for why acquiring him?
That's not that hard.
He sells kits.
SAF wanted him.
City wanted him. I think there's a strong argument that United's leadership would not or could not allow him to suit up across town. I'm a bit shocked he even considered it.
United is reactive not proactive.
Ole seems like a good dude who thinks they can figure out a way to plug him in.
I think any of those answers fit the question, but it's a bit hard to know exactly why unless you're in the board room making the choice.