Harnik said that Stuttgart created enough chances to win at least 2-0 but instead they now face a tricky trip to Belgium needing an away goal.
"We failed to make in count the second half and couldn't score the second," Bundesliga.de quotes Harnik as saying.
"We had a lot of possession in crucial situations, but we were simply too wasteful. We created enough chances, and then we conceded such a stupid goal. It was preventable, so this feels like a defeat."
Harnik has scored seven goals in 29 appearances so far this season.