The problem more than the occasional blunder is that he's awful at everything out of the line. Playing from the back, defending set pieces and crosses, ancitipating long balls, the guy is a bag of nerves and insecurity and now even more, as his confidence seems to be in the lowest point ever. You can be patient and understanding with someone who really wants to stay and shows some commitment at least, but he always comes looking over the shoulder with silly demands and letting his contract to end, so he can do one for me, zero sympathy.
The best keepers in the game are probably earning around 150-200k, maybe with the exception of Neuer, but De Gea seems to want 350 or 400 if not more when he can't improve any of his weaknesses, and his only remarkable quality is under question marks too and won't last forever. And people suggesting to give him a fat contract so he can relax and play better, it's laughable really.
The sooner we start looking for a more complete and influential keeper the better for us, and the same can be said about the centerbacks by the way. It's impossible to build a solid defence without players with strong character and mentality who can operate together to anticipate problems and dangerous situations on the field. All our defenders including this man are snowflakes with lots of limitations and lack any leadership. You just can't build anything around this, it's impossible.