HSV scraped past Oldenburg in the DFB-Pokal first round as the 30-year-old Croatian bagged the winner 18 minutes from time after they had seen Heiko Westermann's opener cancelled out by Sebastian Ferrulli in the first half on Saturday as the Bundesliga outfit triumphed 2-1.
However, Petric knows his side cannot afford another below-par display when they open the new league season at the home of the champions on Friday night.
"That was a genuine warning before Dortmund," he told Kicker.
"If we play in such a way against Borussia, we will go down there."
He added on www.hsv.de: "We made life far too difficult for ourselves.
"You could say that certain things were missing in our game. We had our work cut out against them, and failed to create enough space, and we allowed them too many chances.
"We have a lot of improving to do ahead of our game against Dortmund. They play quick attacking football, so we can expect a lot of that coming our way on Friday night."