So I don't agree with you - so naturally I don't watch enough football ?
Pogba can be disciplined - but players like him and Zlatan get frustrated when things don't go as they want. And thats when they drift into positions where they are more likely to get the ball and influence the game. Which is a good quality of course, but in a 2 man midfield keeping your position is vital. And when you see stats of Pogbas movement in a game, you will notice that he is basically all over the pitch. And I have never said Pogba hasnt got a football brains - of course he does. But he is 24 years old, he has not spent a lot of time playing in a 2 man midfield. This comes with experience, which he lacks. I have seen Pogba leave the man he is marking on set-pieces several times - it's not because he doesnt know what to do, but because he lacks the experience to defend. Many of our attacking players do - it's natural, it requires a set of thinking they are not used to.
As for the Southampton-game. Yes, they ran riot on both the left and the right - but they easily played their way through our central midfield as well - because Pogba and others didn't do their job.