The Slovenia international thanked his former mentor, and maintained that his Inter side are a good squad ahead of their meeting with the Partenopei
The two goalkeepers had crossed paths at Udinese, with 35-year-old De Sanctis enjoying eight seasons between 1998 and 2007 at Friuli, with Handanovic joining in 2004 and leaving for fellow Serie A side Inter at the end of last summer.
The 28-year-old spoke about the help he received from his former team-mate, and also looked forward to Sunday's clash.
"When I arrived in Udine, Morgan helped me a lot, and I thank him again," the Slovenia international told Corriere dello Sport.
"At Inter, we are a good group. But it's too early to say where we can [finish]. Let's take it one match at a time. Beginning with Napoli."
Andrea Stramaccioni's men sit third in the Serie A table, with one league win in their last four, but can leapfrog Walter Mazzari's men with a win.