The Italians fell to a 2-1 defeat at Shakhtar Donetsk in the Ukraine in their Group F opener as Feyenoord suffered a 4-0 home defeat to Manchester City.
"The team has grown in personality," said Sarri of his side whose only European trophy was the UEFA Cup back in the glory days of Diego Maradona in 1989. "In the past we became discouraged when faced with difficulties, while now we react with determination. Psychologically we have something more and we hope to confirm this trend."
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Napoli are riding high in Serie A with a perfect record and 22 goals scored in their opening six games putting them ahead of champions Juventus on goal difference.
"Unconsciously it may be legitimate that we feel more competitive at national and international level," continued Sarri. "Let's see if we can have the same level of determination we show in the championship."
Not even the news of Polish international striker Arkadiusz Milik's expected four-month layoff with torn knee ligaments could dint Sarri's enthusiasm on the eve of the match.
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"Compared to the last year we are more ready to deal with any emergency, because we have more points of reference and a tested game,” he said. "We are very sorry [about Milik] but we can't complain and we must keep moving forward."
And they can count on their vocal home fans in the San Paolo Stadium with travelling Dutch supporters banned from attending over fears of hooliganism.
"Tomorrow [Tuesday] it will be an important and very tough match, the support of our fans will be decisive too," said Sarri. "We will play against a opponent with physical strength and quality. Feyenoord are very dynamic, we'll have to rule the game and keep them far from our penalty area.”
Napoli midfielder Jorginho was similarly confident. "We are aware that a lot will depend on us. Against SPAL we showed improved personality and mentality too," he said of their 3-2 comeback win at the weekend. "Tomorrow San Paolo will push us like always and we hope to offer a superb match."
It will be the first meeting between the two sides and for Feyenoord a first Champions League away match in 15 years.
And the Dutch have injury problems with Brazilian defender Eric Botteghin joining striker Nicolas Jorgenson and first-choice goalkeeper Kenneth Vermeer on the sidelines for the Group F match.
Botteghin suffered a knee injury in the league game at home to NAC Breda on Saturday, which Feyenoord lost 2-0.
"It'll be nice to play here, it's just a pity our fans can't be here too, but we understand and respect the decision," said Feyenoord coach Giovanni van Bronckhorst.
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