Liverpool are the first European winners in 25 years to lose their opening Champions League match since AC Milan 25 years ago.
While the Reds controlled the game, a suspect VAR call on Andy Robertson in the 81st minute allowed Dries Mertens to convert from the penalty spot.
As Napoli held their own, Liverpool committed a second defensive error late in the match. Virgil Van Dijk failed an attempted pass back to Robertson which landed at the feet of ex-Tottenham striker Fernando Llorente to score The Blues second goal.
Deja vu for Liverpool in Naples
Napoli got the better of Liverpool again at home for the second year in a row.
Despite Adrian's heroics and Salah's two near-chances, the Italian side played with more discipline and perhaps a bit more will than Klopp's team.
But if last year's result in Naples serves as any predictor: a loss for Liverpool may turn out to fuel a European championship.
Said Carlo Ancelotti to Jurgen Klopp after the game: “I told him: ‘Don’t you worry, if you lose in Naples, then you end up winning the Champions League.’”