The Biancocelesti can guarantee their participation in continental football on Sunday by wresting fifth place from Udinese with a Serie A victory at Cagliari, should Udine drop points at Inter Milan.
If that fails, Petkovic's men could still earn entry to the qualifying rounds by beating city rivals Roma in next week's Coppa Italia final, but the Bosnian does not want to leave anything to chance.
"This game counts for a lot," he told La Repubblica. "It is the last game of the season and it could allow us to confirm the work we have done all season.
"We have done very well in Europe and we have the opportunity to do the same in the Coppa Italia and in the league."
Lazio travel to the Stadio Nereo Rocco instead of over the Tyrrhenian Sea to the doomed Stadio Is Arenas, after Ivo Pulga's men were forced to relocate due to safety concerns at their newly-built home.
"We cannot take this game for granted," Petkovic added. "The Sardinians have had a good season despite their many difficulties.
"They haven't won their last two 'home' games in Trieste but they have a lot of individual skill in the squad and they will want to show off.
"We need to do everything we can to win the game. Only then can we look at everybody else's results.
"Winning this game would give us a huge confidence boost and would certainly put us in a more relaxed mood ahead of the derby."
Petkovic has several injury concerns including defender Stefan Radu, midfielders Pereirinha, Hernanes and Cristian Brocchi and striker Sergio Floccari, while the season is already over for Cagliari players Francesco Pisano, Nicola Murru, Matias Cabrera, Lorenzo Ariaudo and Gabriele Perico.
Rossoblu coach Pulga knows all too well that Lazio cannot afford to lower their concentration.
"Usually they use a 4-4-2," he told La Repubblica. "But if the newspapers are right then they could rest some players ahead of the Coppa Italia final and use just three defenders.
"I will put my best possible team on the field. Tenth place seemed lost last week but now we still have a chance and we want to play for it right until the end."