De Gea is having a terrific season, as per Football in the Clouds:
Goalkeepers are voodoo. Ask Everton. Pretty much, all of Everton’s under performance in the league is down to their inability to stop the ball. They sit dead last in save % (table below), while last season they were 2nd. Everton have been both, slightly lucky (6th lowest shots on target) and really unlucky (league low save %) this season. Both these metrics should regress but it is too late for it to matter, this season.
On the opposite end of the voodoo spectrum, we have Man United, who are getting away with it, this season. 8th in shots on target against and De Gea bailing them out. This United team are riding the variance of their goalkeeper. Will it last? Crazier things have happened.