Yeray Alvarez was forced to leave the Spain U21 camp on Tuesday after an abnormality was discovered in his recovery from cancer.
The Athletic Club player had been diagnosed with testicular cancer last December but underwent successful surgery and returned to action in February.
However, in radiological control this week an abnormality (adenopathy) was detected which must be treated immediately and in the next few days the player will begin chemotherapy.
The 22-year-old defender, Athletic said, will need an "estimated period of medical leave of approximately three months."
RELATED STORIES
Athletic's statement added: "As was done at the time of his illness, the club is at Yeray Alvarez's disposition and requests respect and discretion for the privacy of the player and his relatives."