Should Inter lose for the sixth time in seven games, they could be mathematically out of the running for a top-five finish.
Conversely, that would mean Lazio would have an "80-90 per cent chance" of playing in Europe again next season, according to Petkovic, who wants his side to build on their 6-0 demolition of Bologna at the weekend and move closer to a place in the Europa League..
"I don't think tomorrow's [Wednesday] game is decisive for us, but it could be for Inter," he said at a press conference.
"If we win, we would have an 80-90 per cent chance of qualifying for Europe via the league, but I don't expect an Inter side in a state of crisis.
"It's important that we take to the field aware that we are improving."
Petkovic does not expect Miroslav Klose to have as many chances against Inter as he did against Bologna, when he scored five times.
"We need to make the most of the few chances we create," he said.
"We are improving all the time. I was pleased with the Lazio I saw (on Sunday) and we still have margin for improvement, but I am confident we will get to the end of the season in top form."
Inter coach Andrea Stramaccioni's hands have been tied recently due to a lengthy injury list which is showing no signs of abating.
"All I can ask of my side is that they give it their best and try to get a positive result at home," said the Inter coach, who has ruled Antonio Cassano out of a return on Wednesday.
"We want to get back to winning ways, like against Parma, despite the absences.
"We are Inter and we always take to the field to win."