Liverpool defender Mamadou Sakho is being investigated by Uefa for an alleged anti-doping violation.
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 provide 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 the equaliser in Liverpool’s 4-3 victory over Borussia Dortmund 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 this summer’s European championship being held in France.