Napoli suffered a 2-1 Serie A defeat to Parma on Wednesday as their five-match unbeaten run came to an end.
They sit seventh in Serie A and cannot finish any higher than fifth with three league games remaining.
Gattuso urged his side to stay focused on domestic matters, and not next month's Champions League last-16 second leg against Barca.
"Our mind cannot be focused on Barcelona now. We concede a lot of goals," the Napoli head coach said.
"We spend a lot of time in the opponent's side of the pitch, but we are not allowed to think that we are the best. No. There is the defensive phase as well.
"Numbers tell us we concede a lot, every time our opponents have chances they bring us problems. It is not about our defenders, but the whole team."
Gianluca Caprari and Dejan Kulusevski scored penalties for Parma, either side of Lorenzo Insigne's spot-kick for Napoli.
Napoli are in action again on Saturday, when they host Sassuolo.