Liverpool suffered an injury blow when James Milner limped off in the opening stages of their FA Cup third-round clash with Everton with a suspected hamstring problem.
The 34-year-old needed extensive treatment after going down in the opening stages but was unable to continue and was replaced by 19-year-old Yasser Larouci after just nine minutes on Sunday.
To lose one of the most experienced and versatile players in the squad will be a huge blow to manager Jurgen Klopp, who had rested a host of his first-team squad for the Merseyside derby clash after a hectic Christmas period.
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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."
Klopp made nine changes to the side that beat Sheffield United on Thursday, including a debut for new signing Takumi Minamino, who joined for £7.25 million ($9.5m) from Austrian side Red Bull Salzburg at the start of the month.
Milner, Adam Lallana, Divock Origi, Adrian and Joe Gomez were the other experienced players in the team. Nathaniel Phillips, Pedro Chirivella and teenagers Curtis Jones, Neco Williams and Harvey Elliott completed a youthful-looking Liverpool line-up.
The Reds played 10 games in 29 days across four competitions in December and early January. That included victories over Monterrey and Flamengo in Qatar to lift the Club World Cup for the first time. They also progressed to the knockout stages of the Champions League and won all six of their Premier League encounters to build up a 13-point lead at the top of the table, seemingly putting them on course for a first league title in 30 years.
The only defeat during that run was a 5-0 thrashing by Aston Villa in the Carabao Cup quarter-final, though it was a complete youth team that took to the field in the Midlands as the senior squad, as well as manager Klopp, were in Qatar for their Club World Cup semi-final against Monterrey scheduled for the following day.
That exhausting run has taken its toll, with Milner becoming the eighth Reds player to join their injury list.
Fabinho, Naby Keita, Dejan Lovren and Joel Matip are among those currently on the treatment table, explaining Klopp's cautious team selection against the Toffees.