MADRID (AP) — Athletic Bilbao defender Yeray Alvarez has a testicular tumor and will undergo surgery next week, the club said Friday.

The Basque team said the tumor was detected in tests conducted a few days ago after the 21-year-old player complained of discomfort.

Team doctor Josean Lekue said at a press conference that although further tests are required, there was reason to be optimistic.

"I'm certain that I'll be fine and will return even stronger," Alvarez said on his Instagram account.

It wasn't clear how long Alvarez will be sidelined.

The central defender made his debut with the senior squad a few months ago and had been praised as one of the team's top players this season.

Athletic is seventh in the Spanish league entering the winter break.

Several players and teams across the league expressed their support to Alvarez through social media.

"Thank you everyone for the messages and for the support," Alvarez said.