I feel that Pogba has a greater impact on the game in a midfield-three vis-à-vis a midfield-two at this moment in time. That's not to say he necessarily becomes a bad player when part of a midfield-two; on the contrary, yesterday's performance was very encouraging. Furthermore, he could become a very good player in a midfield-two in the future when facing high-level opposition. But right now however, I think that the only role which Pogba can perform to a world-class level against high-quality sides is as the most advanced midfielder in a midfield-three.
Quite obviously, Pogba has more freedom when he is the most advanced player in a midfield-three. This particular role, sans the responsibility of having to sit deep and protect the defence, enables Pogba to have a major impact on games by freeing him up to link-up with other players, play aggressive through-balls and also to shoot.
Any player who plays in a midfield-two has to accept the roles and responsibilities of the position. Playing deeper in a midfield-two generally requires greater tactical awareness than playing as the 'free' midfielder in a midfield-three. This usually involves being more adept at covering spaces in defence and being much less likely to constantly roam forward. There is no reason as to why Pogba can't be more tactically adept in the future and as a consequence, there is no reason as to why he can't be a very good player in a midfield-two in the future. Right now however, I think that Pogba just doesn't have that tactical awareness that's needed to be highly effective in a deeper role against top-quality opposition. As success in the 'free' role in a midfield-three isn't as predicated on tactical awareness, it makes sense that Pogba is right now a more effective player there than he would be in a midfield-two.
There's also the issue that for as good a midfielder as Pogba may become in that deeper role, he might always be more effective in the more advanced 'free' role. It could happen that his ceiling as a player in a midfield-two may be as a very good midfielder, whereas his ceiling as the most advanced player in a midfield-three may be as a world-class midfielder. Now, this is an entirely hypothetical situation but at the top level, is the opportunity cost of playing Pogba in a less effective role too great for United to endure? I think that's the most important question when it comes to deciding as to where Pogba should play on the field for this club.