Ter Stegen rued the mistakes made by Ronald Koeman's side, the club conceding three goals in a Champions League home game for the first time since May 2013.
"One thing is individual errors and another is the way you defend. They are different things and you have to value it differently," the goalkeeper said, via Marca.
"We don't get lucky from time to time either. As soon as we get lost they score a goal and this is very hard.
"We are in a phase and we must come out strong, do our homework and work hard to do it. We have to give it a little more from the beginning and come out stronger."
"Until today we have played good games and we have won some very difficult ones. We cannot say that we had a bad phase," he said.
"We had all the illusion to continue and qualify as first but it is what we have. In the Champions League, whether you qualify first or second, they will never have an easy game against any rival.
"We are going to focus on what we have to do. We must improve and nothing more."