Injuries. No manager wants to talk about them, but every team suffers from them. What matters most is overcoming the loss of key players, and Liverpool are discovering that reality right now.
Great players are crucial for any team with ambitions of winning the Premier League title, but it is the squad that ultimately makes the difference, and Liverpool's bid to end their 29-year championship drought will stand or fall on the contribution of their shadow players.
Monday's 1-1 draw at West Ham United was the third time in five league games that Liverpool have dropped points, so the injuries are beginning to bite. Trent Alexander-Arnold, Dejan Lovren, Joe Gomez, Jordan Henderson and Georginio Wijnaldum all missed the trip to the London Stadium -- some with more-serious injuries than the others -- and, all of a sudden, Liverpool don't look like the team that was blowing all opponents away during the final weeks of 2018.
"The team is doing well and we have a tough situation in the moment," manager Jurgen Klopp said after Liverpool emerged from the West Ham clash with a point. "The injuries, I don't know where they come from, or the illness.
"You have difficult situations. It's difficult in the moment, but the boys still fight.
"I don't moan about that, it's just a fact. It's not an excuse. [Milner] came back, but he did not train yesterday -- he came to London by himself because he's infectious. That's all not perfect, of course."
A month ago, Liverpool were riding high at the top of table and unbeaten, but a defeat at Manchester City at the beginning of January, followed by an FA Cup exit against Wolves and back-to-back league draws against Leicester and West Ham, have all combined to dent confidence at Anfield that this might finally be their year in the Premier League.
After the draw at West Ham, Klopp's team have now dropped four points in the past two games and given City the opportunity to reclaim top spot with a victory at Everton on Wednesday. It would be premature to suggest that Liverpool have been afflicted by an early outbreak of title jitters -- West Ham was simply another leggy performance after a tired display against Leicester.
So far, Liverpool are not showing themselves to be weighed down by nerves or the fear of failure. That is not the problem that Klopp has to address. The big issue facing the German is how to fill the gaps in his team when key men are missing because, as we have now seen in successive games, the players who are promoted from the bench when injuries and suspension claim the first choice are not at the same level as those who are missing.
Milner, as reliable as he has been for Liverpool, no longer has the legs to play at right-back, but with Alexander-Arnold and Gomez injured and Nathaniel Clyne surprisingly allowed to join Bournemouth on loan, the former England player is being asked to plug a gap he isn't able to fill, and he was repeatedly caught out of position at the London Stadium. And by playing Milner at right-back, Klopp denies the team the veteran's work rate and experience in midfield -- qualities that were desperately missed against West Ham with Henderson and Wijnaldum unavailable.
Adam Lallana, making his first Premier League start since October, was off the pace and a peripheral figure, aside from his contribution to Sadio Mane's goal, while Fabinho was unable to offer the drive of Henderson or game management of Wijnaldum. Naby Keita was also unconvincing in midfield as West Ham, driven by the impressive Mark Noble and Declan Rice, more than matched the league leaders in the middle of the pitch.
At the back, Liverpool once again failed to keep a clean sheet, having been so resolute and stubborn until the beginning of 2019. Having conceded just eight goals with 12 clean sheets in their first 19 games, Liverpool have now shipped seven goals in their past six games and kept just one clean sheet. The absences of Alexander-Arnold and the oft-maligned Lovren have hurt Liverpool, with Joel Matip unable to form a similarly formidable defensive partnership with Van Dijk as the Croatian had done until his injury.
Still, the good news for Klopp and Liverpool is that their front three of Mohamed Salah, Roberto Firmino and Mane have remained fit and available while other areas of the team have been hit by injuries. But although, between them, they have scored eight goals in their five league games of 2019, they have not put their foot on the pedal as Sergio Aguero has done at City, with the Argentine's hat trick against Arsenal on Sunday taking his tally to five in five league games since the turn of the year.
In fairness to Salah, Mane and Firmino, nobody else has scored a league goal for Liverpool in 2019, so they are doing their bit, but there was nothing left in the tank at West Ham after Mane scored. If they stop scoring altogether, the worry for Liverpool will be that their lack of depth will leave them unable to provide any other answers.