That doesn't take away the fact that he was bad tonight...
He was only poor on the cross, which, honestly, should have never been on target in the first place if we had an actual defense. Even in VV's last season here when he was in complete beast mode he constantly went into no-man's land the way Mats did, but the difference is we often managed to get the ball clear, as opposed to letting a player who's a midget by even our standards get his head on the ball.
The first goal was not his fault by any means. Mascherano is not a CB and playing him there may work against the Elches and the Malagas, but it won't work against tough opposition. Put Pique or Bartra or Vermaelen or Pique or Mathieu, which is literally every CB we have who could have been in that position instead, in that position, and that shot doesn't even take place.
The third goal was, also, down to absurd Barca defending. Of course, it seemed like most of our back line had shut down by then, but ter Stegen could have possibly tried coming out for it, but had he have done that and missed it would have been an even larger mistake. Why the ball was allowed to be crossed in and finished with absolutely zero pressure is what needs to be addressed, not ter Stegen's ability to close his legs when he slides.
Of our entire defense I would say ter Stegen was second best, behind Mathieu. Any blame for this loss should fall on the line that's supposed to be protecting him.