Liverpool face the prospect of being without Virgil Van Dijk for several months after it was confirmed their key defender requires knee surgery - but they are reluctant to rule him out for the season.
News of anterior cruciate ligament damage to the 29-year-old, sustained in a controversial high challenge by Everton goalkeeper Jordan Pickford in Saturday's 2-2 Merseyside derby draw at Goodison Park, is the nightmare scenario for the Reds in their bid to retain the Premier League crown.
"Following surgery, Van Dijk will begin a rehabilitation programme with the club's medical team to enable him to reach full fitness as soon as possible," a club statement said.
Not only does the injury rob them of one of their most influential players for the majority of their Premier League title defence, but also leaves Liverpool with just two senior centre-backs in Joel Matip, who has been plagued by niggling injuries himself, and Joe Gomez.
Jurgen Klopp opted not to replace Dejan Lovren when he left for Zenit St Petersburg in the summer and was content to gamble on utilising midfielder Fabinho as cover.
The damage suffered in his clash with Pickford, for which the Everton goalkeeper escaped punishment due to VAR official David Coote overlooking the tackle after ruling on the Holland captain's offside in the build-up, means Van Dijk will have to have an operation.
However, club sources are unwilling to write off his season just yet, considering the player's previously excellent fitness record, and remain hopeful he can still play a part in the campaign at some point.
It leaves Klopp with an immediate headache as he faces having to field the untested pair of Matip, whose return in the Merseyside derby was his first appearance since the opening day, and Gomez, who was dropped at the weekend after some recent patchy form.
The 23-year-old Nat Phillips, with just one FA Cup appearance to his name either side of two loan spells at Stuttgart last season, is now the back-up option, with the highly-rated but unproven 17-year-old Billy Koumetio another alternative.
Liverpool's defence has looked leaky already this season, conceding 13 goals in five matches, including seven at Aston Villa, with Van Dijk in the back four, so his absence for any length of time would raise real questions about their chances of staying the distance in a title race.
Klopp, however, was keen to quickly throw his backing behind Gomez after a shaky couple of weeks.
"The set-piece (for Everton's first) was unlucky because Virgil was not on the pitch, we sorted it in the game but it was the first corner when he was not on the pitch," he said.
"Joe Gomez played a sensational game, a good game. I am not worried about the boys.
"The problem we have at the moment is that I am not really in a good mood, I have injured players in the dressing room and that is not good."
The news on midfielder Thiago Alcantara, who also went for a scan on a knee problem sustained in the tackle which saw Richarlison sent off late on, is slightly more promising.
Liverpool have to wait for the injury to settle but are optimistic it is not serious, although their £20m summer signing from Bayern Munich is unlikely to feature this week against Ajax.
Liverpool are, in addition, understood to be concerned by the confusing and contradictory messaging emanating from the Premier League in the wake of the VAR controversies in Saturday's game.
The club contacted the Premier League after the 2-2 draw asking for a review of the two major decisions: Pickford's challenge on Van Dijk and the added-time Jordan Henderson goal which was struck off.
Liverpool want to know why Pickford's reckless tackle was not considered for a red card just because the defender had been ruled offside by a replay.
They were also puzzled about the mechanics which saw Sadio Mane ruled offside in the build-up to Henderson's goal, specifically which part of the Senegal forward's body caused the infringement as replays were inconclusive to say the least.
On the Pickford challenge, it is understood that video official David Coote did not check for a potential red card despite having the power to do so and called 'check complete' once he had ruled on the offside.
However, there have since been differing accounts offering explanations as to how both decisions were arrived at and that has led to Liverpool expressing further concerns privately.
While not calling into the question of the integrity of matchday officials, the club believe it is crucial to have more transparency around how and why the outcomes were determined for the integrity of the game for everyone and the safety of players.
In a pulsating derby without fans, Mane and Mohamed Salah netted for the visitors with Michael Keane and Dominic Calvert-Lewin on target for the Toffees.