Jurgen Klopp revealed James Milner suffered a muscular problem in Liverpool's 1-0 FA Cup win over Everton and described the injury as "a big, big blow" for the Reds.
Milner, one of five recognised senior players in the Liverpool starting XI, left the field after just nine minutes of the Merseyside derby, which was settled by Curtis Jones' superb second-half goal.
The injury will see Milner join Naby Keita, Joel Matip, Dejan Lovren, Nathaniel Clyne, Fabinho and Xherdan Shaqiri in the Liverpool treatment room, and leaves Klopp short of cover for Andy Robertson at left-back.
Debutant Yasser Larouci replaced Milner, who appeared to be holding his hamstring, and Klopp praised the 19-year-old defender along with the other young charges who impressed against an experienced Everton side.
Klopp told BBC Sport: "Millie [went] off, which is a big, big blow for us I have to say and it doesn't look too good, a muscle injury.
"But then Yasser coming on and just filling the role.
"It was an incredible game for us decided by a sensational goal from a Scouser. I cannot wish for more.
"It's just a really wonderful night, apart from Millie. The plan was not to play Millie in the last game. The plan was to play Naby but then Millie had to play and today he started again and now that [injury] happened, but we cannot change it."
Nat Phillips and Takumi Minamino made their Liverpool debuts in a team that also featured Neco Williams, Pedro Chirivella, Harvey Elliott and Jones - a quartet of players under the age of 23.
Klopp felt the performance and the result justified the decision not to send any of those who featured out on loan, praising their determination to force their way into his first-team plans.
"You cannot perform like the boys performed today if you think to yourself you should not play in that team," said Klopp. "They have to think that they are ready.
"None of these kids are fine being sent back to the under-23s, but if they were out on loan somewhere that they would maybe play more they would not be here tonight and we would not have a team actually.
"It's already clear they are good enough. What you have to show is respecting our principles. Counter-press is not a proposal; it's the law. And they played brave football.
"Unbelievable performances tonight from the kids but from the adults as well."