The 30-year-old center-back mentioned that he ingested the substance "involuntarily" through medication he was taking to address hair loss after his testicular cancer treatment.

Alvarez was diagnosed with the illness in 2016 and returned to play in 2017 after treatment, but faced a recurrence later that year, making a second comeback in 2018.

"After overcoming my illness, I have been undergoing treatment for alopecia for several years, and after investigating the matter, we discovered that the positive test resulted from my involuntary consumption of a preventive medication for hair loss that contained a banned substance," Alvarez stated in a declaration.

"The disciplinary proceedings are currently in an investigatory phase and confidential, so I am provisionally suspended and am not permitted to make any further public statements."

Alvarez indicated that his positive test occurred after Athletic's 3-0 home loss to Manchester United in the first leg of the Europa League semi-final on May 1st.

The defender also participated in the second leg and two additional LaLiga matches before the season concluded. Alvarez mentioned that he was informed of his positive test "a few weeks ago," and receiving the news was "a very difficult blow."

"I couldn't believe it, as I have never in my life taken prohibited substances," the Spaniard explained.

Athletic Club released a statement expressing that they "regret the human error of Yeray" and will provide support to the defender, but cannot comment further at this time.

The European football governing body UEFA did not immediately respond to a request for comment.