Athletic Bilbao defender Yeray requires chemotherapy after an anomaly was found during a medical examination on Monday.
The 22-year-old has left the Spain Under-21 squad to undergo treatment and is expected to be sidelined for three months, the club confirmed.
"During a radiological check-up done on June 12, the latest of the follow-up tests done to monitor the progress of the testicular tumour that was detected and removed in December 2016, an anomaly (adenopathy) has been detected that needs to be treated and hence, in the coming days the player will undergo complementary treatment (chemotherapy) indicated by the specialists," an Athletic statement released on Tuesday read.
"The player will leave the U-21 Spain national team on June 13 and the estimated time of absence is approximately three months. Just as we expressed when we first found out about his illness, Athletic Bilbao is at the disposal of Yeray and requests respect and discretion for the player and his family."
Yeray, who made his senior debut last September, was diagnosed with testicular cancer in December and immediately had surgery to remove the tumour.
The centre-back returned to first-team action in February and later that month signed a contract extension through to 2022.
He made a total of 35 appearances in all competitions in 2016-17 and earned a place in Spain's U-21 squad for the European Championship finals in Poland this month.