Liverpool defender Mamadou Sakho is being investigated by Uefa after failing a drugs test.
Although the France international, 26, is not officially suspended, he will “not be available” for selection during the investigation, Liverpool said.
BBC Radio 5 live senior football reporter Ian Dennis said Sakho failed a test after the Europa League victory over Manchester United on 17 March.
It is thought Sakho tested positive for a type of fat burner, said Dennis.
The player has until Tuesday to respond with a B sample.
Sakho, an £18m signing from Paris St-Germain in 2013, has played 34 games this season, including 10 in the Europa League.
He scored in Liverpool’s 4-3 victory over Borussia Dortmund on 14 April which secured their place in the semi-finals.
Liverpool, who are seventh in the Premier League, host Newcastle at Anfield on Saturday.
Sakho’s Liverpool team-mate Kolo Toure was banned for six months in 2011 while at Manchester City after testing positive for a weight-loss drug contained within “water tablets” recommended to him by his wife.
A similar ban for Sakho would rule him out of Euro 2016 in France, which starts on 10 June.
BBC Radio 5 live senior football reporter Ian Dennis
“This is a player charge rather than a charge to the club.
“It doesn’t mean Sakho is suspended – he would be eligible today and for Thursday’s Europa League semi-final first leg against Villarreal.
“However, the owners, manager Jurgen Klopp, chief executive Ian Ayre and the player himself all felt it was in the best interests for him not to be available while the process was ongoing.”