Yamal and Athletic Bilbao's Williams both missed the end of the club season with hamstring injuries and their imminent returns will be a big boost to the European champions.
Osasuna winger Victor Munoz is also expected to be fit for Spain's opening game in Atlanta, although none travelled to Mexico for Monday's friendly with Peru and instead remained at their training camp in the United States.
"We regret that they are not here with us, but the medical and physical conditioning staff advised them to stay there," De la Fuente said.
"All three should be available for the next match... they are progressing well in their recovery and are doing very well."
Spain will be one of the favourites at the World Cup in Canada, Mexico and the United States, and the 18-year-old Yamal could be one of the star performers.
After facing Cape Verde, Spain will play Saudi Arabia in Atlanta before finishing their group stage campaign against Uruguay in Guadalajara.
FIFA World Cup 2026
The 2026 World Cup will be held from June 11th to July 19th in the United States, Canada and Mexico. The tournament will feature 48 national teams and will be played in 16 modern stadiums.
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