Peter Stoger conceded Borussia Dortmund were fortunate to overcome Atalanta en route to the Europa League last 16.
Dortmund advanced to the next round 4-3 on aggregate thanks to Thursday's dramatic 1-1 draw with Atalanta in Bergamo.
The German side looked set to bow out on away goals following Rafael Toloi's 11th-minute opener until substitute and captain Marcel Schmelzer equalised with seven minutes remaining.
Speaking afterwards, Stoger told reporters: "Well, effectively, we were a bit lucky. It was only to be expected that it would be difficult, but we certainly made it easier for them in the first half.
"We were not aggressive or physical enough, and we did not apply what we had spoken about doing beforehand. That was not the kind of professional game that you have to play on such occasions.
"We were better in the second half, and the substitutions worked well above anything else. I am really pleased for Schmelzer that he got the goal at the end, as he is our captain and had been out injured.
"Marco Reus showed what good he does for our game when coming off the bench. We had talked to him in recent weeks and wanted to be careful to avoid a relapse.
"We have the right attitude, and the boys have demonstrated that a number of times. Of course, I'm pleased that we are through to the next round, but we have also got a lot to talk about after this match."
Dortmund – second in the table and 19 points behind leaders Bayern Munich – return to Bundesliga action against Augsburg Monday.