The Azzurri trail Juventus by nine points ahead of the seven-game run-in - an advantage the Swiss feels is insurmountable - but victory at San Siro would more or less guarantee Napoli entry to the Champions League group stages.
Milan could be left seven points behind having dropped points in a 2-2 draw at Fiorentina while Napoli beat Genoa 2-0, but Behrami feels both results were misleading.
He told the Corriere dello Sport: "Our only regret is that we are out of the title race.
"We could get closer to Juve but now we are only focused on qualifying directly for the Champions League by beating Milan at San Siro.
"It would be an extraordinary achievement and it would allow the club to plan more confidently for the future.
"It seems like we're favourites because the Rossoneri could only draw in Florence. But that's not how it is - it will be an extremely difficult game. They are in fantastic form."
Napoli head coach Walter Mazzarri has a fully fit and suspension-free squad at his disposal while Milan miss seven-goal striker Mario Balotelli, who earned a three-match ban for abusing a match official in Florence.
Giampaolo Pazzini has recovered from injury and should therefore partner young Italy international Stephan El Shaarawy. The 20-year-old has, however, failed to score in five league outings.
"We will need to show great humility if we are to get a positive result," Behrami added.
"We have to mark all Milan's key men out of the game with a suffocating pressure. That's how we'll make it difficult for them."
Midfielder Kevin-Prince Boateng, meanwhile, has asked to play in his favoured central position after finding himself drifting across Massimiliano Allegri's forward line this term.
He said: "It's important to have a defined role. I don't like changing position every game, from winger to attacking midfielder or to striker.
"The coach has always told me 'It's perfect for me because you can play in five positions'. I reply 'Yes, but it's no good for me'.
"Tomorrow's [Sunday] game is one we have to win to finish second and that is our target for the season.
"It will be a difficult game, even though it's at home, because Napoli are incredibly strong."