Lazio have picked up more than twice as many points in their opening seven fixtures, but the superiority indicated by the Serie A table must now be demonstrated at the Stadio Olimpico, where Petkovic is not taking anything for granted.
"Looking at the league table and given that we are at home then, yes, we are favourites but we need to prove that," he said.
"Milan are a strong side who have been doing well in Europe for many years and the seven points in the league don't reflect their true value.
"They are bound to come to Rome to get the points.
"It is a delicate game because Milan are hurt and they have a lot of pent-up anger and determination to get these points.
"It is going to be an important game for our season."
Nevertheless, Petkovic has every faith in his own side, who he "will always put above our opponents, no matter who they are," and who "must win the game" tomorrow [Saturday] night.
"I hope we can show our true value over 90 minutes," he added at a press conference.
Milan have failed to show their true value so far this season, but they may well have the missing piece in the jigsaw back tomorrow [Saturday].
Pato has yet to play this season and the man predicted to rise from the shadows of Zlatan Ibrahimovic is ready to make his comeback from injury in the capital.
"Pato has not played for a while and he does not have 90 minutes in his legs, which is normal," said coach Massimiliano Allegri.
Giampaolo Pazzini is likely to be preferred over the Brazilian, supported by Kevin-Prince Boateng, who has also been struggling for his form this season.
Allegri is not alarmed by his dip in form, though, saying: "Players cannot be phenomenal one season and then pathetic the next.
"Of course this team can do better, but we need time. Nobody expected us to have 20 points, but nobody thought we would have seven either."
Former Milan player Cristian Brocchi could return to the Lazio squad, but he is unlikely to figure, at least not from the beginning, while Stefan Radu is back in contention too.