Liverpool's upcoming Champions League semi-final against Villarreal isn't their first high-profile meeting with the Spanish side, and nor is it their first collision course with manager Unai Emery in European competition.
Back in 2016, during Jurgen Klopp's first season in charge, Liverpool came from behind to beat the Yellow Submarine in the Europa League semi-final, only to lose to Emery's Sevilla in the final in Basel. However, the Reds' had to face both sides without centre-back Mamadou Sakho after a drug test controversy which rumbled on for years.
The French international was sidelined as the Merseyside club investigated "a possible anti-doping violation," leaving them without a man who played a big part in their European run. The defender would later end up suing the World Anti-Doping Agency (WADA) over the incident, but the resolution came long after Emery had got the better of Klopp and added a third straight continental title to his and Sevilla's trophy cabinet.
It all began with a last-16 game against Manchester United - a fixture which was already notable for being the first ever meeting between the giants in UEFA competition. The Reds had progressed thanks to a 2-0 home win and a 1-1 draw at Old Trafford, but Sakho was found to have failed a doping test after the second leg.
The French defender had played the full 90 minutes of the away leg, partnering Dejan Lovren in central defence, and the pair also played a huge role in the quarter-finals before the alleged offence was made public. With the Reds needing three goals in the final 30 minutes against Borussia Dortmund, both centre-backs were on target in a momentous comeback.
“I wasn't there, but I told the lads in the dressing room at half time we were 3-0 down in Istanbul, and it is unlikely, but it is possible, and so we should try it,” Klopp revealed after the victory. "To come back against the highest opponent like this you need a little bit of luck, but everybody who saw the game will say it was deserved, because at the end the passion was something incredible.”
Barely a week after the game, though, things were turned on their head with the Sakho news. Liverpool acted quickly, confirming the player would be unavailable for selection, but the consequences were far-reaching.
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"It is not that we can do anything to force the situation now, we have to wait," Klopp said upon learning of the failed test. "Until now we did what we thought was right.”
It was a complicated situation for the manager. Sakho reportedly argued he had taken a supplement not realising it contained an unlabelled banned substance, and - as it later turned out - the substance in question was not definitively banned after all.
Shortly after Liverpool made their position clear, UEFA did the same. Sakho would provisionally be banned for 30 days, effectively ending his season and threatening his pace in France's Euro 2016 squad, with a two-year ban to follow if he was found guilty of breaking anti-doping rules.
In such a situation, there's not much a player can do other than wait and hope to be vindicated. No actions on Sakho's part could have sped things along, but the timing was about as bad as it could have been.
The former Paris Saint-Germain defender hadn't always been the most convincing member of Liverpool's back four, but an impressive display away at Dortmund and another goal in the Merseyside Derby had won him plenty of plaudits. He had played 34 times for the Reds that season, as well as a couple of internationals for Les Bleus, but he'd miss out on the fruits of his labour as Klopp's side gunned for European glory.
Not only that, but a place in Didier Deschamps' Euros squad was now out of the question. Sakho hadn't been a lock to start at the home tournament, but a 90-minute runout in the March international break suggested a strong finish to the season would have been enough to get him over the line.
Ultimately, that appearance against Russia in March would end up being his last in France colours for more than two years, and he has added just one more cap to this day. He would also end up leaving Liverpool without playing another competitive minute, after Klopp sent him home from a pre-season tour and later sent him - first on loan, then permanently - to Crystal Palace.
The summer of 2016 saw Sakho's Anfield fate effectively sealed by the arrival of two new centre-backs. Ragnar Klavan ended up being a stopgap, leaving after two seasons, but Joel Matip remains a key man for the Reds to this day.
Before then, though, there was a small matter of a European semi-final without one of the men who had taken them there. Kolo Toure got the nod alongside Lovren, starting only his second game of the knockout run, and the Ivorian was caught out by Denis Suarez as the Spaniard played in Adrian Lopez for Villarreal's first leg winner.
If we know one thing about Klopp teams, though, it's that they have never known when they're beaten. From Dortmund's Champions League quarter-final win over Malaga in 2013 through to Liverpool's triumph over Barcelona six years later, it's taken a lot to break the spirits of the German's teams.
A Bruno Soriano own goal at Anfield levelled the scores on aggregate, and second-half goals from Daniel Sturridge and Adam Lallana completed the turnaround. However, the final was a different proposition altogether.
There's no guarantee Sakho would have made the difference, but Liverpool's defensive frailties were laid bare by Emery's men in a second half comeback of their own. Kevin Gameiro drew the Spanish side level after Sturridge's fine opener, and two quickfire Coke strikes ended it.
That was the end of Liverpool's Europa League story - they haven't played another minute in the competition to this day - but Sakho's story was far from over. His provisional ban ended less than a fortnight after the final, and UEFA opted not to continue with the case later that summer.
The damage was done, though, and in 2019 the Frenchman announced his intention to sue WADA for £13m after claiming the ban caused significant damage to his earning potential. He won substantial but undisclosed damages the following year after clearing his name over "the worst thing you can be accused of".
"Today is a big day for my story," the defender said. "I think it is important for athletes to have this kind of example and to be careful about everything they are taking.”
Klopp wasn't always on the best of terms with Sakho when the pair worked together. However, he too sent a message to his former player once the saga was concluded.
"My initial thought was ‘finally!’" Klopp said in 2020. "We all knew it for a long time already that Mama did nothing wrong. It was a massive, massive blow.
"If you carry around the mark that you did doping, especially when you didn’t do it, that’s massive. So I’m really happy he’s freed of all of these things."
