David de Gea is not just a goalkeeper that any club would let go like that, fine his wages are high but he should be respected to the fullest and for the fact that he still remain loyal to the club like when Madrid came calling he stayed put that shows so much love and dedication to the club and with what I'm seeing I just wish he leaves the club and find a better club within Europe. I won't advise him going to the Saudi league because I see him getting another trophy and making a name for him self ones again, if a better club came calling.
De Gea is no longer in his prime, thus I doubt he'll be able to join a team in Europe and win trophies with them. If the Saudi offer arrives, he should not hesitate to decline it. I'm confident that he'll do quite well there, and that his contributions will be greatly appreciated and acknowledged, as opposed to his current team.
No matter how he makes mistakes I feel is because of the kind of defenders he have around and I see him doing better in another club, Manchester United don't deserve him at all and they'll regret not keeping him if he leaves.
If a certain error persists, you will not hesitate to change the person in charge of that position. In De Gea's case, letting him go isn't a big concern if they can find a stronger successor for him soon. And his departure from the club does not imply that bad luck will follow the team; in fact, they may do even better than before.