Villarreal vs Liverpool: Jurgen Klopp facing defensive crisis following Mamadou Sakho's failed drugs test

Jurgen Klopp faces a selection dilemma ahead of Liverpool's Europa League semi-final clash against Villarreal on Thursday following Mamadou Sakho's suspension for failing a drugs test. The French defender could face a lengthy ban, which puts at risk his participation for his national team in the European Championship finals and for the Reds next season.

The former Paris Saint-Germain star had formed a strong understanding with Dejan Lovren at the heart of the Liverpool back four, which has been one of the main reasons for their strong run in the Premier League and Europa League. The German coach will now have a big decision to make going into the final games of the season, and despite a number of options available, it will be a crucial call ahead of the semi-finals.

Uefa has indicated that Sakho can play the first-leg as no formal disciplinary proceedings have begun, but the Reds have made the defender unavailable for selection until the investigation has been concluded.

"Yesterday, Friday 22nd April 2016, a formal communication was received from Uefa stating that they are investigating a possible anti-doping rule violation by Mamadou Sakho. The player will respond to Uefa on the matter," Liverpool explained in a statement on their official site.

"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."

The absence of Sakho will see one of Kolo Toure, Martin Skrtel or Lucas Leiva occupy the vacant spot alongside Lovren at the heart of the Liverpool back four. The manager will have to decide who is best suited in context of the game.

Kolo Toure

The Ivory Coast defender will bring a wealth of experience to the backline and is sure to give a committed performance throughout the 90 minutes. He has the pedigree of playing at the highest level during his time with Arsenal and Manchester City, but his overzealousness has led to costly mistakes, which makes him a danger for a clash of this magnitude. Toure, however, has played himself back into favour after struggling at the start of the season, and has made five appearances already in the Europa League this campaign. He is match-sharp after making regular appearances in recent weeks and could be the favourite to start against the Spanish outfit.

Martin Skrtel

The Slovakia international has completely fallen out of favour following his return from injury and has struggled to break into the first-team, which has seen him play just 135 minutes of league football since the start of March. Skrtel drew a lot of flak for his performance during Liverpool's 3-2 loss to Southampton, but the defender has proved to be a robust choice on a number of occasions this season. There will be question marks over his fitness and his ability to last the pace for the entire 90 minutes, which could see him reprise his place on the bench, as he did during Liverpool's 2-2 draw against Newcastle United on Saturday (23 April).

Lucas Leiva

The Brazilian midfielder has been used at centre-back by Klopp on a few occasions this season and could be an option on Thursday against the La Liga club. Despite not possessing the build of the former two, he is a good reader of the game and can stifle attacks on most occasions. However, the manager is likely to use Lucas as a midfield anchor owing to the absence of his regular midfielder's Jordan Henderson and Emre Can due to injury.