Former Liverpool defender Jamie Carragher insists Mamadou Sakho's absence is a huge setback for the team ahead of their Europa League semi-final clash with Villarreal on Thursday.
The France international failed a drugs test after taking a 'fat burning drug', claiming that he was not aware the substance was banned.
The 26-year-old defender was subsequently suspended by the Merseyside giants while the investigation is in progress.
"It's a nightmare for the player," Carragher told Sky Sports. "I think he's let himself and the club down badly, because he's become a really important player.
"There's serious questions to be asked of him. He's had a great campaign. He's one of the leaders of the team now, certainly within that run, thinking about how well he played in the Manchester United and Borussia Dortmund games.
"It's now how Liverpool fill that void. For a professional footballer, as in any sport, it's not acceptable."
The Reds travel to El Madrigal in Spain on Thursday for the first leg of their Europa League semi-final clash with the Yellow Submarine.