Liverpool defender Mamadou Sakho failed a drugs test after last month's Europa League game against Manchester United and he has until Tuesday to tell UEFA why a prohibited substance had been taken.
Although Sakho is not yet subject to a UEFA suspension, the club announced Saturday that he will not play for the Europa League semifinalists while the case is active.
Sakho scored one of the goals in Liverpool's 4-3 quarterfinal victory last week over Borussia Dortmund to set up a semifinal meeting with Villarreal.
A fat-burning substance was detected in a drugs test taken after Liverpool's round of 16 second leg at Premier League rival United on March 17, a person familiar with the situation told The Associated Press. The person spoke on condition of anonymity to discuss the substance because the investigation is ongoing.
Sakho played the entire 1-1 draw at Old Trafford which took Liverpool through with a 3-1 aggregate win.
UEFA informed Liverpool on Friday of the "adverse finding in a doping test."
European football's governing body said Sakho and Liverpool have until Tuesday to "request the analysis of the B sample as well as to provide explanations for the presence of a prohibited substance in the players' body."
Discussions between the Fenway Sports Group ownership, led by John Henry, manager Juergen Klopp and Sakho led to the decision to remove the player from consideration for selection.
"A formal communication was received from UEFA stating that they are investigating a possible anti-doping rule violation by Mamadou Sakho," Liverpool said in a statement. "The player will respond to UEFA on the matter.
"The player is currently not subject to any playing suspension. However, the club, in consultation with the player, has decided that while this process is followed the player will not be available for selection for matches."
Sakho was replaced in the team for Saturday's Premier League game against Newcastle by Kolo Toure, who was banned for six months by the English Football Association in 2011 while playing for Manchester City for taking banned water tablets to control his weight.
If Sakho received the same length ban as Toure it would rule him out of the June 10-July 11 European Championship on home soil in France. The sanction would be imposed by UEFA because the test was taken after a European game.
That would be another potential blow for France coach Didier Deschamps, who will already be without star striker Karim Benzema — due to Benzema's involvement in a sex-tape scandal involving his France teammate Mathieu Valbuena.
Although Sakho is not guaranteed to start for France at Euro 2016, he would normally be a certainty to be named in Deschamps' squad on May 12 and would be fighting for a center back place alongside either Rafael Varane or Laurent Koscielny.
With 28 caps, Sakho is an established squad member and popular among the players. Deschamps, who is known for his loyalty, will always be indebted to Sakho for his two goals in a 3-0 win against Ukraine in a World Cup qualifier in Nov. 2013 — sending France to the World Cup.