Lorenzo Insigne had no explanation for Napoli's below-par performance at Shakhtar Donetsk that resulted in a 2-1 defeat.
Having been drawn into the tricky Group F, which also includes Manchester City and Feyenoord, in the Champions League, the Serie A side could have done without a loss in their opening game.
But Shakhtar were good value for the three points as they netted through Taison and Facundo Ferreyra before Arkadiusz Milik's penalty made a fight of the closing stages.
Napoli had chances to grab an equaliser, but Insigne conceded his side made errors across the 90 minutes that they must now address.
"We knew that Shakhtar were a good side as they have quality players in attack," he told Mediaset Premium. "I honestly don't know what we got wrong and why we played like this.
"During the week, we will discuss it with the coach and see what mistakes we made so that we can correct them.
"Perhaps we believed a little too late that we could come back, but we had chances and didn't make the most of them. We had the opportunities to equalise. It's a pity.
"We've got to get our heads down and work hard to improve on the errors we saw today."
Maurizio Sarri mixed up his team selection, leaving star man Dries Mertens on the bench, but Allan – also a substitute – had no issue with the decision.
"We are all ready to play and therefore all ready to be on the bench, too, as there are many games," he added.
"I respect the decision of the coach. There are many quality players and we've always got to be ready."
18. Juni 2018
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17. Juni 2018
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|Brazil 1 Switzerland 1: Zuber pegs back below-par Selecao|
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|Germany follow trend of poor starts for World Cup champions|
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|Southgate keen to unleash his bold new England|
|Konta fumes as Barty takes Nottingham title|
|Kante hails Pogba´s match-winning contribution|
|Germany struggle with generation gap|
|Neymar to make first competitive start since February in Brazil´s World Cup opener|
|Kane gunning for Ronaldo hat-trick heroics in England opener|
|Balotelli would fit in at Marseille - Raiola|
|Captain Ki happy to get tough on South Korea|
|Senegal call up Mbengue with Ciss ruled out|
|Mickelson celebrates at scene of Saturday meltdown|
|Germany 0 Mexico 1: Lozano stuns woeful reigning champions|
|Chandimal to face hearing after pleading not guilty to ball tampering|
|Griezmann ´must do more´, says Deschamps|
|Gasquet shows his class on grass to end Chardy´s chances|
|Courtois: Belgium outsiders not World Cup favourites|
|You can´t compare Ronaldo and Messi – Adrien Silva|
|Kiwis name seven uncapped players for England Test|
|Lorenzo tells critics: Remember MotoGP wins next time I struggle|
|Maaloul singles out ´future of English football´ Alli as main threat|
|Poland v Senegal: Nawalka waits on Glik|
|All Blacks draft in Riccitelli and Hemopo, Barrett not risked for third Test|
|Palermo to appeal play-off result after ´illegal game´|
|Once a year is not enough! - Alonso loved Le Mans bow|
|Colombia v Japan: Kawashima eager to right the wrongs of 2014|
|Russia v Egypt: Salah poised to derail hosts´ juggernaut|
|South Korea swapping kit numbers to confuse Sweden|
|Neuer, Ozil start for Germany in World Cup opener|
|Costa Rica and Serbia serve up plenty of the spectacular in sunny Samara|
|Alonso edges closer to ´Triple Crown´ with Le Mans glory|
|Costa Rica 0 Serbia 1: Kolarov stunner seals points|
|Belgium are better than Argentina – Gomez and Panama ignoring Iceland result|
|Messi, Ronaldo or Neymar would keep the goal - Evra defends Pogba|
|Two in a row for dominant Lorenzo in Barcelona|
|Maradona denies racism claim|
|Resilient Krunic breaks Tour duck|
|Federer wins Stuttgart Open on his return|
|Injured Vunipola to miss England´s series finale in South Africa|
|Ibrahimovic not involved in Sweden´s World Cup plans, insists Andersson|
|Sri Lanka captain Chandimal charged with ball tampering|
|Portugal will not underestimate Morocco, vows Adrien Silva|
|Messi is not Maradona, he does not win a World Cup alone - Crespo|
|Hourcade resigns after Pumas´ series defeat|
|Second place is worthless - Rivaldo admits pressure is on Neymar and Brazil|
|South Africa character made up for mistakes - Erasmus|