The veteran custodian expressed his hopes that the Partenopei can still qualify for Europe's premier club competition despite their recent setback against Bologna
Only a three-point deficit separates the southern Italian side from Udinese and next season's Champions League qualifiers.
De Sanctis believes that the Partenopei will bounce back from last weekend's shock setback and will take care of business against the Tuscans on Sunday.
"It was a bitter blow losing 2-0 away to Bologna," De Sanctis told Il Corriere dello Sport.
"Nevertheless, I choose to stay positive because the last few days have shown that motivated teams can distort the rankings from what they may be today."
As things stand, Napoli lie in fifth position, but De Sanctis was optimistic that the Partenopei will pounce on any slip-ups committed by their third-place rivals in the last round of fixtures.
"We'll think only of winning our match against Siena, and hopefully await favourable results from Catania-Udinese and Lazio-Inter," he continued.
"The drawback [against Bologna] was indeed ugly, but the season has not yet been completely written.
"The Napoli project is a serious one which has had the chance to grow internationally. What will happen now may be impossible to predict, but the club will always continue growing stronger and more organised."