Ter Stegen was involved in Eric Garcia's 10th minute dismissal for Barca.
He later said: "We didn't understand each other well in that situation, I feel bad for Eric, it hurts him. And we played 80 minutes with one less player. We didn't understand each other. Something happened that shouldn't happen to anyone.
"These are things that happen in football. The defeat hurts, because with 10 on the field we stood up and tried to get a point."
He continued: "We stood up for ourselves. And the second goal came at an undeserved moment. In the end you are left with the bad feeling of not having picked up any points.
"We knew they were going to push hard. We wanted to get together and it worked out pretty well for us to a certain extent, but they scored a goal without much play.
"In the Champions League you always have to win. That's the objective. We'll try to win the next game at home."