Spain forward Nico Williams was substituted in the 37th minute of Athletic Club's 1-0 home loss to Valencia on Sunday with an apparent hamstring injury, raising concerns about his availability for the World Cup. Williams was replaced by his brother Inaki Williams at San Mames stadium.

Athletic Club did not immediately release details on the nature of the injury. Williams had already missed several weeks earlier this season due to a separate injury.

"He was limping a lot. He hadn't felt that type of pain before," Inaki Williams said. "It's concerning considering the moment we are right now. Let's wait and hope for the best possible scenario."

The 23-year-old is a key figure in Spain's attack alongside Barcelona's Lamine Yamal, who is also currently sidelined with an injury. Spain coach Luis de la Fuente is scheduled to announce a 55-name preliminary World Cup squad on Monday.

Spain were drawn in Group H of the expanded 48-team tournament alongside Cape Verde, Saudi Arabia and Uruguay. The World Cup is hosted jointly by the United States, Canada and Mexico this summer.

Spain currently have the best World Cup odds to win the tournament ahead of France, England, Brazil and Argentina.