There's nothing wrong with his mentality, he has elite mentality. One of the problem is that he's adjusting to a new role, similar to what happened to him at Juve last season. This United team doesn't have enough genuine match winners & he doesn't have the kind of classy CM partners he had at Juventus, football is not black and white, just because you cost 89m, then you'll just come into a team that has struggled for 4 yrs and everything starts working smoothly, honestly, some of you don't understand just how complicated these things are.
Another thing with Pogba is Jose Mourinho. Maybe this is just me, but I don't think Jose knows for sure what he wants to do with Pogba, and even against Bournemouth when he lost his head and was hoofing it, what stopped Jose from giving him specific instructions on what to do? Jose just stood at the touchline and watch him do it for 45 minutes, how about a rollicking, a talking to etc. Maybe he's giving him freedom to make things happen, but you do need to give players specific instructions when they're losing their way. And the obsession with 4-2-3-1, I'm sorry, this formation is never going to get the best out of Pogba, even though he can still do it at a good level.
The third is Pogba himself, always caught in two minds, there's an indecision about his game that he needs to sort out. The best central midfielders in the game know what they'll do before they receive the ball, it's something he has not mastered though he did that quite well at Juve, he'll need to improve on this and also on his general awareness. So, nothing wrong with mentality, application just has to be better, a top player by every standard and I'm very happy we have him, we'll be calling him world class if he was playing at Real Madrid.