The Reds led 2-0 thanks to goals from Mohamed Abdellaoui and Christian Pander, but were made to sweat for all three points in their Group B encounter after Oleksiy Kurilov pulled a goal back.
"We tried to be clinical, particularly when attacking, and we managed that in the first half, but after conceding in the second half, we became unsettled," he said on his club's website.
"Maybe we are just lacking a bit of experience in Europe in that instance. We have got to learn from that.
"But in the end, we earned the win and that shows our quality."
Goalkeeper Ron-Robert Zieler, in his first season as a professional, echoed Slomka's words by demanding more consistency in future.
Hannover's next group fixture is at home to Copenhagen on October 20, and he wants his team-mates to focus for the whole of that game.
"If we want to survive in Europe, then we have got to play two good halves and not just one," he said.
"Nevertheless, we have got the three points and we have just got to be happy with that."