Alvarez, 30, is a centre-back who has been battling testicular cancer since 2016. After treatment, he made a comeback in 2017 but suffered a recurrence later that year. He returned to play in 2018.

Alvarez said he took the banned substance involuntarily through medication for hair loss. He has been taking treatment for alopecia for years, and a test found that the banned substance was in the preventative medicine.

Alvarez's positive test came after Athletic Club's 3-0 home defeat by Manchester United in the Europa League semi-final first leg on May 1st. He played in the second leg and two more LaLiga matches before the end of the season.

Alvarez was told he tested positive a few weeks ago and found the news to be a tough blow. He denied ever taking prohibited substances before.

Athletic Club expressed their support for Alvarez, calling it a "human error". However, they cannot comment further on the matter at this time.

The European football governing body, UEFA, has not made a statement about the case yet.