Ter Stegen made a very poor judgment call leaving his position. He is human and made a mistake, something he rarely does. Don't be quick to shout at him, be quick to shout that we just as a squad can't seem to come back from any mistakes made.
All the same, Ter Stegen is one of the best goalkeepers we've had in a long time and I wouldn't change him for the world even after tonight.
Secondly, am I the only one on here feeling like Koeman just isn't cut out for this job? He cannot seem to get the squad to deliver at any hint of resistance. That and reading his managerial stats, there's nothing much impressive about it.