Athletic Bilbao defender Yeray Alvarez has been provisionally suspended for the "unintentional" use of a banned substance, which he used to treat alopecia.
The 30-year-old Spaniard tested positive for a prohibited substance following Bilbao's 3-0 defeat to Manchester United in the Europa League semi-finals on May 1.
Alvarez explained that he had been using the medication as part of his treatment for hair loss after being diagnosed with testicular cancer in 2016.
Having made 257 appearances for the La Liga club, Alvarez expressed disbelief upon receiving the test results, describing it as a "very hard blow" to endure.
"After reviewing the situation, we determined that my positive test was due to the unintentional intake of a hair loss prevention medication containing a banned substance," Alvarez stated in a social media post.
"The disciplinary process is ongoing and confidential, so I am provisionally suspended and not permitted to make any further public comments."
"I deeply regret this situation, but with the club's support, I am working on my defense and hope to return to play as soon as possible."
UEFA informed BBC Sport that they would refrain from commenting until a final disciplinary decision is reached.
An Athletic Bilbao statement expressed, "Athletic Club fully supports Yeray Alvarez during this time and has complete confidence in the authorities responsible for resolving the case."
"Once the proceedings are concluded, Yeray Alvarez and Athletic Club will provide all necessary and relevant explanations."