Napoli is comfortable to play at home. Therefore, as you said, the possibility of rotation is high. Nevertheless, as I said, since these teams will also play in the champions league, maybe he can use the excuse of preparing for that match and use a squad accordingly in this match. In a way, we can think of it as a preparation for this match. So Napoli will prepare accordingly. Maybe they don't want to concentrate too much on this match because they want to eliminate their opponent in the Champions League.
Napoli tend to feel very comfortable when playing in this season, from Serie A to Champions League, from Italy to other European countries. Comfortable feeling when playing at home can help them play better a little bit but it won't be helpful if Napoli are not strong enough to convert advantage from home stadium and fans to goals and win matches.
I think Napoli will be cautious in this match and not show all of their cards against Milan. While Milan needs points badly for the champions league spots and they need to win this match. While Napoli can rest some players because they already won the title.
It's smarter for Napoli if they won't show all they have in the Serie A match with AC Milan. They will have three big matches (two ones in Champions League) with AC Milan and they would consider matches in Champions League are more important than the one in Serie A.
I don't think Napoli have anything to hide after all impression they made in this season but would it be more careful to avoid showing threats and all ability before two very important matches in Champions League. Maybe about two-third of season, AC Milan could not find good way to beat Napoli but after a match in Serie A, they will find something. Napoli must be very cautious about it.