Brendan Rodgers admitted Leicester City only had themselves to blame after crashing out of the Europa League.
An end-to-end encounter saw four goals shared in the first half in Naples, where the hosts led 2-0 courtesy of strikes from Adam Ounas and Eljif Elmas.
However, the Foxes responded through Jonny Evans and academy graduate Kiernan Dewsbury-Hall's first goal for the club.
Elmas struck again early in the second half and, despite Rodgers' side having chances to get themselves level once more, it proved decisive.
Leicester had began the night top of Group C, but they ended it in third place and headed into the Europa Conference League after their defeat was compounded by Spartak Moscow's 1-0 win at Legia Warsaw, with the Russian club topping the table.
"When I was told what had happened they had to pick me up off the floor but ultimately it's our responsibility and it's my responsibility.
"We're devastated but you have to do your own job to qualify and you can't keep leaving yourself needing three or four goals to win games.
"I'm devastated for the supporters. They've been brilliant and it was a horrible night with the weather and everything."
Asked if winning the Europa Conference League was now the priority, Rodgers told BT Sport: "I've got to be honest, I don't even know what the competition is.
"In all fairness, I was focused on the Europa League and winning this group, or at the very least finishing second, so with all due respect to the competition, I'm not sure what it is.
"But I'm sure I will find out soon enough."
Leicester were without a number of first-choice stars due to Covid, but Napoli - ravaged by injury - weren't at full strength themselves, boss Luciano Spalletti without star players Kalidou Koulibaly, Victor Osimhen, Lorenzo Insigne and Fabian Ruiz, while Frank Anguissa and Stanislav Lobotka were also absent.
Leicester made a bright start and should have taken an early lead. Dewsbury-Hall charged down the left flank all the way to the by-line and his far-post cross was met by Timothy Castagne inside the six-yard box, but goalkeeper Alex Meret somehow made a smothering save on his goal line.
Napoli went straight up the other end and scored. A mistake by Tielemans, although the midfielder was not helped by the pass from Caglar Soyuncu, resulted in Ounas finding the corner of the net with a low shot across Kasper Schmeichel after four minutes.
It saw the midfielder mark his first start for the club since April 2019 with a goal.
Elmas doubled Napoli's lead 20 minutes later. Andrea Petagna strolled through the middle of the Leicester defence before unselfishly teeing up Elmas for a tap-in.
Evans immediately pulled a goal back, smashing home the loose ball from eight yards out after James Maddison's free-kick had struck a defender and landed in his path.
With 33 minutes on the clock, Leicester were level. Another free-kick was only half-cleared by the hosts and Dewsbury-Hall volleyed the ball into the bottom corner of the net from the edge of the penalty area.
Leicester's hard work in getting back into the match was almost undone at the beginning of the second half, with only a fine save by Schmeichel by his near post preventing Ounas from getting his second goal of the night.
However, the Foxes did find themselves behind again after 53 minutes when Elmas grabbed his second goal of the night, controlling Giovanni Di Lorenzo's low cross before giving Schmeichel no chance with his close-range finish.
Immediately at the other end, Di Lorenzo inexplicably passed the ball across his own penalty area where Maddison intercepted before, with the goal at his mercy, letting Napoli off the hook as his shot clipped the outside of a post and went wide.
As the final whistle went in Naples, the Foxes were almost handed a dramatic reprieve in Warsaw, as Legia were awarded a penalty deep into injury time.
An equaliser would have seen Leicester go through despite their defeat, but the spot-kick, from ex-Tottenham youngster Tomas Pekhart, was saved.