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The Blaugrana have been disappointing since the restart – they surrendered a two-point lead in La Liga to finish five adrift of eventual champions Real Madrid – leading to speculation over head coach Quique Setien's future.
However, Napoli boss Gattuso still considers overcoming Barca to reach the quarter-finals in Lisbon to be one of the toughest challenges his team could face.
"They are used to winning and I know that when you get used to winning you always want to do it. Tomorrow we have to climbEverest," Gattuso said on Friday.
"In recent months I have heard many jokes, like 'Barcelona are not in shape', but if you go and watch them there are some players who put on a show.
"We know it will be a very difficult match for us. We know what kind of team awaits us and we know we will have to make a great performance, both technically and tactically, and maybe that won't even be enough.
"We know we're facing great champions. You need a great game to come out with a good result."
Gattuso provided a positive update on the fitness of Lorenzo Insigne, who limped off during last weekend's victory over Lazio with a groin injury.
"Yesterday he trained at 50 per cent. Today he was at 100 per cent with the team," said Gattuso.
"I want to know he feels 100 per cent tomorrow. If he is not, he will not be on the pitch from the first minute. If he is at 100 per cent he will play and help us make a great game."