He's not comfortable / confident with the ball at his feet ?
What the feck have you been drinking / smoking / injecting ?
You're mistaking manager instructions with weakness.
Mourinho wants him to peg the ball up the pitch.
His weakness is crosses where there is a question if he should come off his line or not.
I agree with that, but even in the brief moments when he does get to show his footballing skills, he isn't comfortable as the very best sweeper keepers - he mishits quite alot of passes despite not being asked to contribute that much with the ball. Definitely a gulf in class to say VDS, on the ball.
Like you said, if he played under a Guardiola, I am sure he would brush up more on distribution and he'd become better at it. However even under LVG a guy who favoured passing out of the back, there was some improvement but nothing eye-catching about his distribution during that period.
Can you envisage him coming out and heading the ball, chesting it down.. like a Neuer or Ederson? because that is the type of thing you'd have to do in a Pep side. At the moment, he hasn't for me shown he can do that yet - but nor is it impossible for him to gain these skills.. Valdez and Neuer were not natural ball players, and were coached into being a good fit whereas Hart was a write-off. I don't think De Gea is that bad, just compared to his peers.. it isn't a strength at this moment in time.