Rafa Benitez's side escaped with a goalless draw in Wales on Thursday in the Europa League last-32 first leg.
Napoli host Swansea next Thursday with all to play for.
"The goalless draw is not a bad result but it could have been better," Hamsik said. "It's clear that we will have to play a great game at home against Swansea in order to progress.
"We knew they were strong and they made life difficult for us.
"Now we must focus on our game against Genoa [on Monday] and try to take three points.
"Only after Monday's game will we think about our return leg against Swansea."
Pepe Reina will replace Rafael in Napoli's goal after the Brazilian goalkeeper sustained a right knee injury against Swansea.
Rafael will undergo surgery in his home country in the next few days.
The good news for Benitez is that midfielders Jorginho and Gokhan Inler as well as striker Jose Maria Callejon are available having missed last weekend's 2-0 triumph at Sassuolo through suspension.
Napoli are third in the standings and hold the last Champions League qualifying spot.
They are six points clear of nearest rivals Fiorentina.
Genoa coach Gian Piero Gasperini is hoping his players can show the same fighting spirit that earned them a deserved point against Udinese last time out.
The Grifoni hit back from two goals down to draw 3-3 against Udinese last Sunday.
"Our game against Udinese was one of our best performances of the season," Gasperini said. "It's good to see that we are improving.
"I hope my team can show the same determination against Napoli."
Genoa have lost just one of their last five league games and are 11th in the standings.