The match will be both club's first involvement in this season's competition, with the visitors having finished fourth in La Liga last season, while Napoli clinched third in Serie A.
Rafael Benitez could give on-loan Swansea City striker Michu his Champions League debut, while Gonzalo Higuain could also start up front after returning to training last week following an extended break after the World Cup.
The Italian side have been in strong form during their pre-season friendlies, defeating Barcelona and PAOK, while only narrowly losing to Paris Saint-Germain.
Meanwhile, the visitors have not reported any injuries in the run up to the match, but will be playing their first competitive match since Ander Herrera's departure for Manchester United earlier this summer.
Oscar de Marcos and Borja Viguera could feature up front, while Aritz Aduriz has been in good goalscoring form during pre-season, netting a hat-trick against Borussia Monchengladbach earlier this month.
This will be Bilbao's first involvement in the Champions League since the 1998-99 season, when they reached the group stage, while Napoli will be looking to reach the competition proper for the sixth season in a row.
Athletic Bilbao: DLLWD
Possible starting lineups
Napoli: Rafael; Albiol, Britos, Andujar; Henrique, Maggio, Hamsik, Jorginho, Insigne; Michu, Higuain
Athletic Bilbao: Iraizoz; Balenziaga, San jose, Laporte, Gurpegi, Iturraspe; Muniain, Rico, De Marcos; Aduriz, Viguera
Sports Mole says: 3-1