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A conundrum sprouted from Liverpool’s defeat to Manchester United: What has 14 legs, but just three Premier League goals?

With time ticking and faith flickering, Jurgen Klopp turned to his bench. He had already called upon Adam Lallana midway through the second half in an attempt to re-energise the midfield. Now, he needed firepower, after the front three had found their fuse dampened.

On came Gini Wijnaldum, deployed as a right back, a position he has rarely played. Soon after, Dominic Solanke. The Dutchman tried to help his side in transition, while the 20-year-old looked to provide a mere presence in the penalty area. The hope was he could put something, anything, onto the ball and guide it towards the net.

It did not happen. That is not to the young striker’s detriment, who continues a steep learning curve in his first senior campaign in the Premier League. Much like his more experienced, exalted team-mates up front, he did not receive sufficient service.

Yet his arrival – likewise the emergence of Lallana and Wijnaldum from the bench – pointed to one of the few weak points Liverpool possess at the moment.

(Image: Michael Regan/Getty Images)

Lallana has not scored this season, albeit it has been a campaign ravaged by injury. Wijnaldum’s last away goal came in May 2015, for PSV Eindhoven, the day after Steven Gerrard said goodbye to Anfield.

No surprise Solanke is yet to register a first-team goal in England given his tender age, either.

If the front three fail to flourish, what happens next?

Mohamed Salah, Sadio Mane and Roberto Firmino have scored 68 goals between them this season, a remarkable feat. Their blank at Old Trafford represented just the second time Liverpool have found the scoresheet in the league this season without one of those adorning it.

After that trident’s piercing contribution, Emre Can has scored six, Alex Oxlade-Chamberlain four and Trent Alexander-Arnold three.

Liverpool score for fun, turning several games into a fiesta, but it so often depends on whether the front three have answered their invitations. For the majority of this season, they have.

Not so at Old Trafford, and so, a nagging concern over the lack of back-up bubbled back into consideration.

Just three goals sat on the bench against United; one apiece for Wijnaldum, Joel Matip and Jordan Henderson. There is not much further in reserve, with Danny Ings yet to score a goal for Klopp, and both Daniel Sturridge and Divock Origi on loan.

With the futures of the latter two in question – Sturridge is likely to leave, while it is difficult to see how Origi has produced the sufficient development at Wolfsburg to challenge for a first-team place – Klopp faces a summer of potential reshaping in terms of options up front.

Solanke, Ings and Origi are all young, and can all still improve. But none of them function as a direct replacement for Firmino. None of them match his goal return or his creativity, of course, but nor can they replicate what the Brazilian does for Klopp’s team. He initiates the press, he creates space for others, he defends from the front.

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Firmino is not only unique at Anfield, but in Europe. No striker can do what he does, and few are so key, so in-tune, to how a team functions. If the 26-year-old is not present up front, Liverpool’s whole set-up would be different; efforts to replace him without altering what happens around him would be akin to filling the tank with chicken soup and still trying to drive the Grand Prix. The space is filled, but it requires a total rethink of how to run the race.

Liverpool have a problem, then. It is a nice problem to a degree, as they possess a player so vital, so individual and so downright good. But a problem nevertheless. What would happen if Firmino was to become injured for a crucial Champions League tie, and how would that impact the entire make-up of the side?

(Image: Andrew Powell/Liverpool FC via Getty Images)

Finding a like-for-like replacement would be both difficult and expensive. If there is a player who can do what the South American does – even to a lesser extent – then he will cost money, and he will be hard to convince a bench on Merseyside is a sensible career move. Even if he is happy to fight for a starting spot with Firmino, spending a vast sum on a player who improves options but not the first-team would represent a risk.

Not that Klopp is reluctant to spend in the summer. “When the player is the right player it is not important how expensive he was,” he said on Friday. “He needs to fit in the team, that’s why we paid the price for Virgil (van Dijk).”

Liverpool will strengthen at the end of the season, and a goalscorer in reserve should be considered.

But when Firmino is so good, it could give Klopp a bad headache in the summer. The defeat to Manchester United demonstrated where the Reds can still improve.

The biggest conundrum of all may just be how.