BILBAO, Spain — Europe’s midseason soccer transfer window is open this month. For teams battling relegation, and potential financial ruin, it brings a last, vital chance to strengthen a leaky defense or fortify a goal-shy forward line ahead of the second half of the season.

Among those teams is Athletic Bilbao, which is in 17th place in La Liga, the 20-team Spanish league, and already on its second manager this season.

Financially, Athletic, a century-old power on Spain’s northern coast, has little in common with the clubs that find themselves in a similarly grim position. The team is flush with cash, with about 200 million euros, or about $228 million, in reserves, and has access to another 90 million euros that — if deployed to lure talent from soccer’s global marketplace — would probably go a long way toward firing it up the league standings and out of danger. But unlike almost every other team in the market, Bilbao cannot spend those millions on just anyone.

Athletic is a throwback. Tradition dictates that it can only field players who were born in the Basque Country — territory that includes seven provinces that stretch from northern Spain into France — or who moved to the region in their youth and learned to play soccer here.