Mudryk said he had “never knowingly” taken banned substances, Chelsea said in a statement.
The FA did not comment on the situation when approached by The Associated Press and Chelsea did not say if he had been suspended. Mudryk was last included in a Chelsea squad as an unused substitute on Dec. 1.
“Both the Club and Mykhailo fully support The FA’s testing program and all our players, including Mykhailo, are regularly tested,” Chelsea said. “Mykhailo has confirmed categorically that he has never knowingly used any banned substances. Both Mykhailo and the Club will now work with the relevant authorities to establish what has caused the adverse finding.”
The 23-year-old Mudryk has made nine appearances for Chelsea this season, but played only once in the Premier League.
The FA's anti-doping regulations state it is a player's “personal duty to ensure that no Prohibited Substance enters his body”.
Mudryk was one of the most sought-after players in Europe when he became Chelsea’s latest big money signing under American owners Todd Boehly and Clearlake Capital.
It was widely reported that he was a target for Arsenal before he completed a move from Shakhtar Donetsk to Chelsea. He was given an 8 1/2-year contract, which is unusual in soccer.
But he has so far struggled to live up to the hype and his price tag, making just 26 league starts.
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James Robson is at https://twitter.com/jamesalanrobson
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AP soccer: https://apnews.com/hub/soccer