Heritage has always been part of Athletic’s fabric: The team’s primary jersey has vertical red and white stripes, showing two of the three Basque colors (green is the third); Basque berets can be found for sale (and on heads) around the redesigned Estadio de San Mamés; and coaches at Athletic’s academy frequently instruct the players in Basque instead of Spanish. Before Thursday’s match, the stadium announcer listed the starting players in Basque, and when Athletic and Barcelona — the flagship team of another autonomous community, Catalonia — played in the Copa del Rey final this year, both teams were penalized because their fans whistled during the playing of the Spanish national anthem.

“We have our own language, culture, music, sports and everything that makes a country different from another,” Imanol Amuriza Saratxaga, a 32-year-old fan of Athletic, said Thursday.

Fans and club officials are aware of the drawbacks and criticisms of their policy. From a business standpoint, it clearly limits the club strategically; Athletic is essentially reducing its player pool to a population roughly similar to that of Connecticut. And from a social and cultural perspective, there are some who find the policy to be exclusionary, even offensive. In a time of increased migration and cross-border movement in Europe and elsewhere, is it appropriate to reject anyone out of hand?

Urrutia, the club president, said the policy had the flexibility to grow with the times. For example, he said, immigrants or refugees who come to Bilbao with young children, or who later have children, can enroll them in Athletic’s academy. Race is not a restriction either, Urrutia said: Iñaki Williams, whose mother is Ghanaian and whose father is Liberian, was born in Bilbao, plays for Athletic and scored two goals on Thursday.

Anyone, even the child of an American expatriate in Bilbao, can play for Athletic, Urrutia said, as long as the person grows up as part of the Basque community. It is that sense of identity — of belonging to the group — that is the root of the policy more than animus against anyone else.

Amorrortu, the sporting director, said he believed that a child needed to be part of the academy by age 14 or 15, at the latest, to truly absorb Athletic’s identity. Having so many players come through the academy feeds the club’s link to the wider Basque community, too, which is why matches for Athletic’s youth teams or reserve team can draw crowds of 10,000 or more.

The policy “makes the people feel part of the team,” said Amuriza, the fan, “because you always know someone close to one or more of the players, and every child in the Basque Country knows they can one day play in San Mamés.”