Dortmund hosted Bochum on the back of a shock 5-1 defeat at the hands of VFB Stuttgart, and they started on the wrong foot after the visitors took an early 2-0 lead in the first half.
"The supposed scenario of a 3-0, which could have happened, was simply because we defended very, very badly. It was very, very poor defending," the coach told a press conference.
"We have to develop in this area and defending in depth is always a huge topic. I hope that we improve as quickly as possible because there's no other way," he added.
Sahin praised the performance of new signing Guirassy, who headed home before the break and scored in the second half, as the key to his side's victory.
"We are all very happy that he is with us. And I'm not just happy that he scores goals, but also how he presents himself. I've said that before. And I thought he was also one of those who was clear-headed at half-time.
"That's why we brought him in, so that he can help lead here, so that he's not just on the pitch but also off it."
Dortmund are temporarily second on 10 points, two behind leaders Bayern Munich. They host Celtic in the Champions League on Tuesday before travelling to Union Berlin in the Bundesliga four days later.
