Not all players arrive as the finished article. At his age, and relative experience (only really Serie A, which let's face it is behind at least La Liga and the Prem) we were (over) paying for what he could become, not what he was at the time.
All these posts basically same thing: he has the attributes/tools to be one of the very best, but isn't there and doesn't do it consistently.
I know real life and football don't parallel all the time, but the above is basically all you learn in manager training. How to take team members and shape what they are into something better for the team. In the end, some of it comes down to the individual, but I'd strongly argue the majority of time it's how much effort and how effectively the manager works on development.
Is Pogba better under Mourinho? I'm not sure. He looked at his best when he basically had no defined 'job' and could just be a footballer. Some managers would acknowledge that, encourage it and build around it. Others find that too scary, as it doesn't fit into their predefined boxes of what a midfielder 'should' look like.
Unfortunately we have a manager that was never a footballer, who I don't personally believe loves football, rather loves winning. And he'd rather have XI automatons that precisely do whatever it is he's planned than create an environment whereby a team can express themselves.
Watch where Pogba goes after Jose, either with us or elsewhere. He will be an absolute star.