Stuttgart dazzled with an exceptional performance, overwhelming Dortmund on their home turf and handing them their first loss in the Bundesliga this season.
"You can't perform like that as Borussia Dortmund. That was the first time with me. That was a non-performance - it has to be said very clearly," Sahin told DAZN.
"Sobering. Absolutely deserved from the first to the last minute. Very, very bad."
The former Turkey international admitted that their planned strategy ahead of the match had failed.
"You don't see that coming, to be honest. We had a session yesterday. We felt like we had trained well - the guys were really sharp. We played through everything again and trained through," he said.
"No matter what tactics and so on: If the basics aren't right on a day like this, then something like this happens. And for us it was nothing at all from the first to the last minute today."
Dortmund are eighth and trail leaders Bayern Munich by five points and Sahin said they must quickly regroup and refocus before their next match against VfL Bochum at home next Friday.
"I can't hide anything that was clearly visible on the pitch. We have to face up to that and put a stop to it. I never want to see a face like that again," he said.
"We'll analyse it hard - as it should be between men - and draw the right conclusions. It's happened, now it's on to Friday. I'm sure we'll perform differently. Because I can't accept that," Sahin added.