Buoyed by an impressive performance in last week's goalless first leg, the Premier League side are confident they can shock their highly rated opponents.
Napoli, who sit third in Serie A, have lost only one home game all season in all competitions, yet Canas insists belief is high among Swansea's players.
"We got a really good result in the first leg, though we would have liked to have scored," he told the club's official website.
"Now we are in Naples and we hope to get a good result to advance to the next stage, which would be a massive achievement for this club.
"We are not the favourites but we have a big chance to win and we will try our best to do that.
"I expect the game to be different as they are at home in front of their fans, but we need to have the same will and determination. If we do that then we can achieve a lot of things."
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