Giovanni Simeone made up for a Leo Ostigard error in a 1-1 friendly draw with Girona, with Napoli then winning on penalties.
Victor Osimhen was ruled out by a slight muscular injury and sat in the stands in what was a reshuffled side put out by coach Rudi Garcia.
A terrible Ostigard back-pass was intercepted and allowed Stuani to open the scoring for Girona after just 13 minutes.
Michael Folorunsho was brought down and Simeone converted the resulting penalty to equalise.
Chucky Lozano played from the start, a signal that the potential transfer to LAFC has been abandoned, as the MLS club only offered €10m for the Mexico international.
Alessio Zerbin and Karim Zedadka wasted opportunities and it ended 1-1, so was decided on penalties.
Alex Meret was the hero of the hour with two saves, while Giacomo Raspadori, Andre Frank Zambo Anguissa and Piotr Zielinski converted from the spot.