UEFA has opened disciplinary proceedings against Mamadou Sakho following his failed drugs test.
The Liverpool defender decided not to have his B sample tested in relation to the doping violation and has now been provisionally suspended for 30 days by European football's governing body.
The France international opted against a further sample being tested ahead of Tuesday's deadline after his initial sample revealed traces of a fat-burning substance.
UEFA said in a statement on its website: "Disciplinary proceedings have been instigated against Liverpool FC player Mamadou Sakho, following the UEFA Europa League round of 16 second-leg match between Manchester United FC and Liverpool FC (1-1), played on March 17.
"The proceedings relate to a violation of the UEFA anti-doping regulations, after the player failed a doping test at the above-mentioned match.
"The player did not request the analysis of the B sample. UEFA has now suspended the player provisionally for 30 days until a final decision is taken by the UEFA control, ethics and disciplinary body.
"The date of the disciplinary hearing will be announced at a later stage."
Liverpool play the first leg of their Europa League semi-final at Villarreal on Thursday evening.
Sakho was not on the team's flight on Wednesday lunchtime as it is understood the club and the player have jointly agreed he will remain unavailable for selection during this process.
Sakho had also been left out of Saturday's draw with Newcastle after the anti-doping violation emerged last week.
Liverpool have refused to comment on Sakho's situation as allegations relate to the player only and they are unlikely to change this stance until the defender and UEFA have resolved the matter.
All parties would likely welcome a swift outcome.