By Alasdair Mackenzie
Dries Mertens scored from the penalty spot with eight minutes remaining after Andy Robertson tripped Jose Callejon inside the box.
Napoli substitute Fernando Llorente added a second in stoppage time to put the result beyond doubt.
In the other Group E game, Salzburg made an impressive start with a 6-2 victory at home to Genk. (Reporting by Alasdair Mackenzie Editing by Toby Davis)
"Charms strike the sight but merit wins the soul." ~ Alexander Pope