Liverpool's big week has been given an unwanted injection of tension by Mamadou Sakho, who has been sidelined by the club while he is investigated by UEFA over a possible doping violation.

France centre-half Sakho, who has been a key player in recent weeks, was left out of the team that drew 2-2 against Newcastle at Anfield on Saturday and is being sidelined by the club, with the first leg of their Europa League semi-final against Villarreal looming on Thursday.

Jurgen Klopp's side, who surrendered a two-goal lead against a Newcastle team managed by their former Champions League-winning coach Rafa Benitez, are also pursuing the top-five finish in the Premier League that would guarantee Europa League action next season.

"There's nothing to say about missing Mamadou," Klopp said.

"There's a statement and that's all we can say at the moment. We'll know more next week probably."

Sakho, whose involvement for France at Euro 2016 is now in doubt, is expected to ask for his B sample to be tested, with the deadline for the request set for Tuesday.

Sakho's absence -- he is technically still available, so it is Liverpool's decision not to select him rather than UEFA's directive -- could hardly have come at a more difficult time for Klopp, who will also discover this week whether Divock Origi has a chance of returning to action this season.

The Belgian striker, again one of the in-form players who has helped Liverpool creep up the Premier League table of late, is awaiting a decision on the ankle ligaments he damaged in the 4-0 victory over Everton that preceded Saturday's disappointment against Newcastle.

If the medics conclude on Wednesday that Origi will need surgery to correct the damage, then he will be ruled out not only of the climax to Liverpool's season but also Belgium's challenge in Euro 2016.

Origi has contributed 10 goals this season from only 15 starts, and his absence for Liverpool would be an even more bitter blow for Klopp if Daniel Sturridge were not in such fine form.

The England striker, who has seen his campaign ravaged by injury, contributed a superb opening goal against Newcastle in the first minute and might also have earned a penalty when Cheick Tiote's desperate challenge halted Sturridge's dangerous run into the penalty area with the score at 2-1.

Klopp, who will look to Sturridge primarily to establish a first-leg lead against Villarreal on Thursday, said: "Ninety minutes is no real problem for him now.

"Before it was 60 minutes here, 30 minutes there, but he is in the best shape he has been in for a few months. For us, that's very important."

Meanwhile, Newcastle boss Benitez admitted his half-time team talk had the desire effect on his relegation-threatened team.

Benitez is always reminded of the 2005 Champions League final, when Liverpool came back to win against AC Milan from 3-0 down at the break, whenever the subject of his former club come up and he was asked what he said to his players at the break at Anfield.

"I have answered this question a lot of times," said Benitez, whose team are one point from safety with three games remaining.

"We knew the first half was not the best and we needed to react so I told them similar to what I said in the past: 'Nothing to lose'.

"We had to fight and show more character and I think the players had a fantastic reaction.

"Liverpool were playing well and it was difficult to stop them.

"But this was different (to Istanbul). That was a final, one game, and now it is a race in which we have to keep going.

"We have to do our job and see what happens with the other games but at least we showed character and the belief is still there, which is crucial for us."