The two teams meet tonight.
"Even if Napoli are not at the top of their form, they still have what it takes to cause us problems," said Cosmi.
"They seem to be on their way back up now and are still in the fight for third place. It is going to be a very tough game tomorrow against a strong opponent.
"We will not rotate the squad, as there are two matches after this in which we'll be forced to. At this stage of the season it's silly to change tactics and I do not see my players losing their way.
"Without doubt going six games unbeaten is a wonderful run and I had hoped to get these results with such quality football. There is satisfaction, but at the end of the day the table still says we're third from bottom."