de Gea has been poor for Spain and Man Utd since halfway last season.
I actually took a look at the statistic at the end of last season. I looked at expect goals against (xGA) and compared it the number of goals allowed. I also included how many saves they made for each goal they allowed. This is what I found:
David De Gea - xGA 42,89 - 28 goals allowed - difference of 14,89. 4,11 saves per goal allowed.
Jan Oblak - xGA 34,95 - 22 goals allowed - difference of 12,95. 4,95 saves per goal allowed.
Marc-André ter Stegen - xGA 40,51 - 28 goals allowed - difference of 12,51. 3,50 saves per goal allowed.
Alisson - xGA 37,48 - 28 goals allowed - difference of 9,48. 3,89 saves per goal allowed.
Samir Handanovic - xGA 37,33 - 30 goals allowed - difference of 7,33. 3,10 saves per goal allowed.
Manuel Neuer (16/17) - xGA 18,30 - 13 goals allowed - difference of 5,30. 3,92 saves per goal allowed.
Gianluigi Buffon - xGA 17,87 - 14 goals allowed - difference of 3,87. 3,21 saves per goal allowed.
Keylor Navas - xGA 32,45 - 31 goals allowed - difference of 1,45. 2,68 saves per goal allowed.
Loris Karius - xGA 14,68 - 14 goals allowed - difference of 0,68. 2,21 saves per goal allowed.
Hugo Lloris - xGA 33,61 - 35 goals allowed - difference of -1,39. 2,45 saves per goal allowed.
Thibaut Courtois - xGA 31,13 - 34 goals allowed - difference of -2,87. 2,24 saves per goal allowed.
Ederson - xGA 22,32 - 26 goals allowed - difference of -3,68. 2,23 saves per goal allowed.
Gianluigi Donnarumma - xGA 36,78 - 42 goals allowed - difference of -5,22. 2,17 saves per goal allowed.
I took the numbers from 2016-2017 for Neuer though, as he almost didn't play last season. I think these numbers reflect what I've said so far pretty well. It's really close between De Gea, Oblak and ter Stegen. I remember being surprised that Neuer's numbers weren't better, but I think it's right to say that he's regressed quite a bit in recent years. Definitely not the Neuer that we saw a handful of years ago.