Roberto Firmino came off the bench and scored a 91st-minute goal to earn Liverpool a 2-1 win over Monterrey and a spot in the Club World Cup final.
Jurgen Klopp opted to name Firmino and Sadio Mane among the substitutes, while Jordan Henderson started at centre-back with Virgil van Dijk - one of the Reds' two first-team centre-backs in the squad - absent due to illness.
Liverpool looked like they would be able to cope without two of their electric front three when Naby Keita put them ahead in the 11th minute, but Rogelio Funes Mori – the twin brother of former Everton defender Ramiro – hit back three minutes later.
Klopp was forced to send on Mane and Firmino in a bid to avoid extra time and the Brazilian turned home fellow substitute Trent Alexander-Arnold's low cross six minutes after coming on to book a meeting with Flamengo on Saturday.
Liverpool started brightly and moved in front when Salah's exquisite throughball was latched onto by Keita, who guided a cool finish beyond Marcelo Barovero.
Monterrey equalised when Allison parried Jesus Gallardo's volley straight to an unmarked Funes Mori and the striker swept home his 100th goal for the club.
The Liga MX side looked dangerous on the break and Alisson got an important touch to a cross from Dorlan Pabon with Funes Mori lurking, while Barovero did well to stop Keita rounding him before half-time.
Liverpool continued to dominate possession but, with the CONCACAF Champions League winners testing Alisson through a Pabon free-kick and looking more threatening in attack, Klopp sent Mane on for Xherdan Shaqiri.
Monterrey boss Antonio Mohamed and Klopp were shown yellow cards in the 77th minute as the tension rose and Firmino soon entered the fray for Divock Origi.
The Reds had struggled to create chances in the second half but when Alexander-Arnold sent a low cross to the near post, Firmino got in front of Miguel Layun and saw his effort squirm past Barovero to snatch the victory.
What does it mean? Liverpool need Van Dijk for the final
Putting Henderson in the centre of defence in the absence of Van Dijk made Liverpool look extremely vulnerable on the counter-attack. With Flamengo boasting a formidable attack of Everton Ribeiro, Giorgian de Arrascaeta, Bruno Henrique and Gabriel Barbosa, the Reds will hope the return of the Netherlands international can provide some much-needed stability.
Firmino the key
He may not have matched the goal tallies of Mohamed Salah and Mane this season, but Firmino found his scoring boots when Liverpool needed it. He had just six touches after coming on and one of them sent the ball past the impressive Barovero to keep the Reds' hopes of winning the competition alive.
Klopp handed Shaqiri his fifth start of the season and the Switzerland international failed to impress. His only shot came in the fifth minute and was dragged well wide, while he failed to create a single chance for his team-mates.
Key Opta Facts
- Liverpool have reached their second FIFA Club World Cup final, becoming the first English team to reach the final of the competition twice.
- Monterrey have lost both of their FIFA Club World Cup semi-final matches, losing against English opposition both times (also vs Chelsea in 2012).
- Alexander-Arnold has registered 20 assists in all competitions during 2019, more than any other Premier League player.
- Since the start of the 2017-18 season, no player has more assists for Liverpool in all competitions than Salah (31, level with Firmino).
- Liverpool became the first Premier League team to play matches on consecutive days since November 2001, when both Manchester United and Arsenal did so.
- Keita has now scored as many goals in his last three Liverpool appearances in all competitions (three) as he managed in his first 41 matches for the club.
Liverpool will take on Flamengo in the final on Sunday, while Monterrey have a date with Al Hilal in the third-place play-off.