Spain forward Nico Williams had to be substituted before halftime in Athletic Club's 1-0 loss to Valencia at home on Sunday, prompting concerns ahead of the World Cup.
Williams appeared to suffer a hamstring injury and was replaced by brother Iñaki Williams in the 37th minute at San Mamés stadium.
Nico Williams had already been sidelined for several weeks earlier this year because of another injury.
Athletic did not immediately release details about Sunday's issue.
"He was limping a lot. He hadn't felt that type of pain before," Iñaki 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 Nico Williams has been a regular in Spain's squad, playing in attack along with Barcelona's Lamine Yamal, who is also injured.
Spain coach Luis de La Fuente is set to announce a 55-name pre-list for the World Cup on Monday. La Roja were draw in Group H with Cape Verde, Saudi Arabia and Uruguay in the expanded 48-team World Cup hosted by the U.S., Canada and Mexico this summer.
Information from The Associated Press was used in this report.