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."
|Lunin rejected ´3,000 offers´ to join Real Madrid|
|On your bike! Iceland physio sent home after cycling accident|
|Not just Real Madrid interested in Fekir, says Aulas|
|France frustrated by officials again as All Blacks complete sweep in style|
|Jose Enrique reveals brain tumour surgery|
|The referee was a disaster, fumes Matic|
|Lakmal to captain Sri Lanka in Chandimal´s absence|
|Alli back in England training|
|McIlroy, Watson plotting moves as Harman leads Travelers Championship|
|Tearful Neymar is a joyful boy, says Jesus|
|Ruiz: Coutinho has the same quality as Neymar|
|Kovacic announces intention to leave Real Madrid|
|Red Sox edge Mariners in 34-hit extravaganza|
|Marquez asks Mexico fans to ´let them rest´|
|Fabregas hopeful of Spain recall|
|Milinkovic-Savic plays down Juventus speculation|
|Rashford inspired by Ronaldo|
|Messi is not a leader, he is no Ronaldo, claims Petit|
|Better than Ballack? Hodgson backs Loftus-Cheek to shine against Panama|
|Svitolina and Garcia suffer upsets|
|Walker responds to Ferdinand criticism|
|Djokovic still hungry for more after reaching major milestone|
|Ellis drop goal earns Castleford dramatic win|
|Nigeria hero Musa unsure on Leicester City future|
|X marks the spot - Granit´s rocket sets World Cup first|
|Xhaka and Shaqiri follow the script as Serbia are derailed|
|Chandimal appeal against ban rejected|
|Real Madrid sign teenage goalkeeper Lunin|
|Serbia 1 Switzerland 2: Shaqiri strikes late to clinch crucial comeback victory|
|Suspended Srna joins Serie A side Cagliari|
|Cilic sets up Kyrgios showdown as Djokovic reaches milestone|
|Kudla up next in Federer´s Halle ´fight´|
|Assistant Farbrace to take charge of England for T20 games|
|Hummels hoping to be fit for Sweden clash|
|Hend surges clear as Fleetwood struggles again in Pulheim|
|Giant-killers Mexico embrace sudden weight of expectation|
|Roma complete €17.25m Kluivert deal|
|Pakistan name strong group for Zimbabwe tour|
|Choc Horror: Boullier insists McLaren do not hand out Freddos as bonuses|
|Neymar: Not everyone knows what I went through|
|Poland v Colombia: No time to dwell on regrets, warns Aguilar|
|Musa shows Messi how it´s done to set up critical Argentina showdown|
|Nigeria 2 Iceland 0: Musa brace boosts Super Eagles & Argentina|
|McLaren and Renault open to Ricciardo talks|
|We must win, we have no other choice - Maaloul|
|Germany will respond against Sweden, insists Low|
|´The best among all Asians´, South Korea trust in Son against Mexico|
|Stalin´s bunker an unlikely site for Russia´s World Cup charm offensive|
|England v Panama: Rashford tipped to start as Southgate´s side aim for last 16|
|Erasmus wary of ´wonderful´ Cipriani|
|Hummels to miss Germany-Sweden, confirms Low|
|Murray accepts Eastbourne wildcard|
|St Petersburg madness lines up Neymar and Brazil with heroes of the past|
|Martinez happy to be ´boring Belgium´|
|Japan v Senegal: Squeaky clean Group H rivals eyeing huge step towards last 16|
|Argentina team the worst in history - Ardiles|
|Ravanelli upsets wife by taking Arsenal job...in Kiev|
|Federer moves a step closer to 10th title in Halle|
|Brazil 2 Costa Rica 0: Coutinho and Neymar score in stoppage time to snatch win|
|Andersson: Sweden embracing big occasion as bug sweeps through team|
|LeBron in ´good dialogue´ with Cavaliers|
|Ronaldo will always be the best, Fonte claims|
|Southgate criticises press for England team leak|
|O´Mahony: Series win in Australia would be up there with Grand Slam|
|Khedira thrilled with Can capture, staying with Juventus|
|Samuel open to Higuain-Icardi swap deal|
|It´s just a game - Milligan disappointed by Kruse criticism|
|From drug-testing lab to gastro pub: Sochi happy not to forget dark past|
|Ronaldo will do what is best for him, says Carvajal|
|Kohli: India will feel at home in England|