The two teams, who are unbeaten and level on points at the top of the league, meet in Turin tomorrow night.
Marchisio, whose late goal saw the Bianconeri defeat Siena two weeks ago, believes that a win over Partenopei would dent their confidence.
However, the Italian international stressed that the title will not be decided by whoever emerges victorious.
"This is the big match of the moment," the 28-year-old told La Gazzetta dello Sport. "Winning isn't important for the Scudetto, as there is still far too long to go.
"However, on a psychological level the three points could take the fizz out of Napoli. It'd hit their confidence and stop the atmosphere getting too fired up. Napoli improve year after year and if they were to gain confidence then they could cause us problems.
"I'll be honest, the first weeks of the campaign would suggest the title is a two-horse race. We are the best teams, the most organised and complete. Juve and Napoli also play the most convincing football. Having said that, there are always dangers lurking around the corner, as Inter, Milan, Lazio and Roma must be respected."
Juve are unbeaten in the league since May 2011.