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."
|Nadal too good for Mayer at Australian Open|
|Jones taking unlikely inspiration from City in United´s title pursuit|
|I had one foot out of the Australian Open – Wozniacki revels in fightback|
|Beware the nearly beaten in Melbourne – Wozniacki could match Kerber, Li|
|Rallying Tsonga outlasts Shapovalov in five-set epic|
|Wozniacki saves two match points in dramatic Aus Open comeback|
|Kostyuk, 15, ready to spend after Melbourne run|
|Thomas turns down Celtics´ video tribute|
|Moyes unaware of Chelsea interest in Carroll|
|Verdi wants Bologna stay amid Napoli links|
|Ronaldinho retires – A shining light of Brazil´s 2002 World Cup-winning team|
|Ronaldinho retires – a World Cup and Ballon d´Or among Brazil great´s prizes|
|Celtics great White dead at 71|
|World Cup glory and Bernabeu magic - Ronaldinho´s best moments for Barca and Brazil|
|Trust the Process – 76ers only behind Warriors and Cavs in merchandise sales|
|Ligue 1 Review: Balotelli at the double as Payet shines and Malcom toils|
|Pato still dreaming of World Cup selection|
|FA Cup Review: Iheanacho makes VAR history, kit confusion reigns|
|Wallaby saves wallaby: Farr-Jones in Sydney Harbour Bridge rescue mission|
|2016-17 UEFA Study: PSG, Man City drive record-breaking spending|
|Conte packing in homework as Chelsea games stack up|
|2016-17 UEFA Study: The headlines in transfers, wages and revenues|
|2016-17 UEFA Study: Manchester United record highest revenue but largest debt|
|Monaco stay perfect, Murcia overcome Oldenburg|
|Iheanacho scores English football´s first VAR goal|
|Materazzi not surprised by Coutinho transfer fee|
|Ronaldinho retires - Brazil and Barca great plans 2018 farewell|
|Jags RB Leonard Fournette uninjured in three-car accident|
|India revenge would heal Proteas´ scars - Elgar|
|Koziello swaps Nice for Cologne|
|Simeone ready to unleash Costa and Vitolo alongside Griezmann|
|Head coach Cooper to leave Stade Francais|
|Silverwood plays down talk of replacing Bayliss|
|Lambert ´never thought´ he would return to Premier League|
|Wenger ´very aggressive´ towards referee Dean, says FA report|
|Reported Arsenal target Malcom starts for Bordeaux vs Caen|
|Valverde not making big deal of Dembele´s latest Barca blow|
|Costly day for Kohli as Proteas close in on series win|
|Date set for Afghanistan´s maiden Test|
|Returning McIlroy ready to end winless run|
|In-form Short leaves coach Kirsten needing dental treatment|
|Arsenal will have a ´hard time´ signing Malcom|
|Ronaldo and Madrid nominated for Laureus awards|
|Nantes´ Diego Carlos brands Chapron clash ´historic´|
|This is not important - Conte unconcerned if substitutions upset Hazard|
|Conte ´doesn´t think´ Chelsea have pursued Sanchez|
|Cobblestone climb part of 2019 Tour de France Grand Depart|
|Emery hints at Neymar return against Dijon|
|Joshua labels Parker claims ´fake news´|
|´Emotional´ Fleetwood honoured by Seve Ballesteros Award|
|Tears for Kvitova as Bouchard sets up Halep meeting|
|Mkhitaryan key to United-Sanchez deal, claims Raiola|
|WATCH: Sabalenka wins point using racquet throat|
|Federer, Djokovic and Zverev among Australian Open first-round winners|
|Kubica gets reserve role as Williams hire Sirotkin|
|Shambolic Stars out after Sixers thrashing|
|Wales pick uncapped Worcester wing Adams in Six Nations squad|
|Tevez: CSL spell was a holiday|
|Mignolet considering Liverpool future|
|Laidlaw among several returning stars in Scotland Six Nations squad|
|´Fairy tale´ 2017 motivation for Federer|
|Fred waiting for Guardiola call amid Man City speculation|
|Dominant Federer makes light work of Bedene|
|Halep faces nervous wait over ankle injury|
|Inter confident but patient over Rafinha|
|Kohli fined after umpire outburst|
|Djokovic hits back at Australian Open boycott suggestions|
|De Grandhomme lifts Black Caps to win in Taylor´s 200th ODI|
|Giggs had counselling after Old Trafford exit|
|Durant continues dominance over Cavs as Warriors win|