It’s funny how opinions can change so quickly in football. Two and a half years ago, Chelsea could barely contain their laughter long enough to get the deal over the line when they accepted PSG’s £50million bid for David Luiz. Fast forward to 2017, and the Blues must be fearing how they’ll cope without their floppy-haired defender at the heart of a backline that has the healthiest goals conceded record in the Premier League.

No doubt, few Chelsea fans were delighted to see Luiz head for PSG in 2014 - even after his infamous role in Brazil’s crushing 7-1 defeat to Germany in the semi-finals of the World Cup - and equally few were disappointed by his return to Stamford Bridge last summer.

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Yet, throughout his first spell at Chelsea, the South American was a cult figure of comedy, controversy and criticism, the epitome of everything ranging from the ridiculous to the sublime. In unexpected contrast, his second spell has been one of consistency, composure and leadership, slotting so perfectly into a system that has produced an eight-point difference in the title race this season.

You know the world is changing when the Premier League is besotted with three-man defences and Luiz is the lynchpin showing the rest how it’s done. Nonetheless, that’s the position the English top flight now finds itself in; for all his prior indiscretions, the Brazilian is perfect for the ‘libero’ role - naturally athletic enough to sweep behind those either side of him, yet talented and confident enough on the ball to become the extra man in midfield.

Suddenly those marauding runs and ranging passes from central defence once deemed so naively hazardous are not only beneficial, they’re encouraged in the formation Antonio Conte plays.

That being said, it’s not Luiz’s offensive efforts that have stood out this season, excepting his voluptuous free kick against Liverpool. He’s actually averaged the second fewest created chances, the fewest passes and the fewest forward passes of himself and back three accomplices Cesar Azpilicueta and Gary Cahill.

Rather, he’s been an integral defensive rock at the Chelsea defence’s beating heart and the success in which he’s managed to shed preconceptions of cavalier naivety is highlighted by two key statistics - 112 clearances, 5.4 per match being the most of any Chelsea player this term, and zero defensive errors.

Whilst Eden Hazard and Diego Costa may grab the headlines for their contributions at the more glamorous end of the pitch, there’s a strong case for Luiz being Chelsea’s most important player this season. He’s taken the return to west London in his stride and whether Conte’s campaign-defining switch to 3-4-3 would have been possible without such a unique defender at his disposal remains wide open to debate.

Which makes Luiz’s latest injury scare all the more concerning for the Premier League pace-setters. Coinciding almost too perfectly with Gary Neville’s premonition of injuries to key players being one of the few factors that could suddenly derail Chelsea’s title bid, Luiz took photographs of himself in hospital on Sunday after being seen limping during a 1-1 draw with Burnley at Turf Moor. It’s claimed he’s been struggling with a knee problem for some time, caused by Sergio Aguero’s horror tackle back in December.

"To win 14 or 13 games on the bounce on the way in, you're still relying upon Chelsea losing three or four matches and winning 13 and 14 games. I don't think we've got the trust in the other teams to do that. The fact that Europe is starting again for many other teams as well makes it more difficult. Chelsea are strong favourites, it could happen, but Chelsea would have to have injuries to key players. That is the one thing that could be a problem to Chelsea."

Of course, any team with John Terry wallowing on the bench can never be too worried about an injured centre-back, yet there’s a reason Luiz, Azpilicueta and Cahill have been preferred this season. Terry’s played in back fours for his entire career and if there’s one attribute Chelsea’s first-choice defenders all share, it’s a smooth turn of pace - something Terry's always lacked, but especially at the age of 36. His limited speed has often forced Chelsea to play in a certain way, but Conte’s moved on; and just as Luiz seemingly suits his set-up perfectly, Terry is simply a poor fit.

The next obvious candidate is Kurt Zouma, a raw, lump of physical brawn who improved at an incredible rate during his first two seasons at Stamford Bridge. He’s got the athleticism to match Chelsea’s regular back three - in fact, he vastly enhances it - but he’s also made just one Premier League appearance - a two-minute substitute outing - since an anterior cruciate ligament injury at the end of last term. With Chelsea now amid the business end of their title bid, can the 22-year-old handle the pressure of replacing a defender as important as Luiz whilst catching up with the seasoned strikers he'll face between now and the end of May?

Nathan Ake represents the last remaining alternative, but once again, the Dutchman doesn’t quite convince. It’s experience that has made Luiz such an intrinsic part of Chelsea’s defensive game this season, yet Ake’s got just 39 Premier League appearances to fall back on, largely courtesy of loan spells with Watford and Bournemouth.

And make no mistake about it, Chelsea’s expected coronation is by no means guaranteed. They may be eight points ahead of a chasing pack constantly cannibalising itself in the battle for Champions League qualification, but a lot can happen in the space of 13 games. Just as the ingenuity of introducing 3-4-3 gave Chelsea a cutting edge in the title race, another event could quickly swing momentum back the other way.

Luiz’s injury (although not yet believed to be long-term or season-threatening) could well provide that spark which switches the tide, disrupting Chelsea’s rhythm at the back and eating away at the resolute defence Conte’s game-plan is now built upon - the Blues have won by more than two goals just twice since beating Everton 5-0 in November and although Jose Mourinho's recent swipes have been labelled as simply sour grapes by many, Chelsea are first and foremost a counter-attacking side.

But, Conte does have three options who could do just enough to halt any decline, even if they can’t quite mirror Luiz’s vitality. So, who should the Chelsea gaffer choose as Luiz’s direct replacement?

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