Alisson Becker has returned to training.

Alisson Becker has returned to training following a hamstring injury.

Caoimhin Kelleher has impressed in Becker's absence; who should Liverpool manager Arne Slot choose as his number one? Watch Liverpool against Manchester City live on Sky Sports on Sunday, kick-off at 4 pm.

Alisson Becker is nearing full fitness following an injury, which means Arne Slot will soon have to choose between the Brazilian and his understudy Caoimhin Kelleher - so which goalkeeper should the Liverpool boss opt for?

Alisson tore his hamstring in the closing seconds of Liverpool's 1-0 win over Crystal Palace on October 5 and has just recently resumed light training, with Slot stating that he and his medical staff will take a careful approach to recuperation.

"We're careful with everyone when it comes to muscles," added the Dutchman, who declined to say if Alisson will be in contention for Liverpool's Super Sunday match against Premier League champions Manchester City.

"He is back in training with the goalkeeper coach rather than with the team, but that is expected soon. Let's wait and see where he is in terms of match fitness; he's improving but not quite ready."

Slot does not need to rush Alisson back into play, as Liverpool has gone unbeaten in all competitions during his seven-week absence and is currently leading the Premier League and Champions League charts.

Not only that, but Kelleher has once again played at such a high level while filling in for Alisson that there is a legitimate argument that he deserves to keep his spot in goal even when the Brazil international has fully recovered from injury.

What do the analytics tell about the two shot-stoppers' relative strengths and limitations, and what other considerations will Slot and Liverpool's new goalkeeping coach Fabian Otte consider when making this difficult decision?

And there are no more extreme pressures for a goalkeeper than a penalty shoot-out, with no Liverpool 'keeper having saved more spot kicks than Kelleher's six in such situations - the Reds have also won all four League Cup shoot-outs he's been involved in - while the Irishman scored the decisive penalty in the Reds' dramatic 11-10 shoot-out win against Chelsea in the 2021/22 Carabao Cup final before being named player of the match against the same opponents in last season.

Not only that, but Kelleher was at it again on Wednesday night, saving Kylian Mbappe's penalty in Liverpool's 2-0 Champions League win over Real Madrid - as well as spot kicks from Southampton's Adam Armstrong, who scored on the rebound, and Finland's Joel Pohjanpalo this month.

Alisson also helped Liverpool defeat Chelsea in a penalty shootout to win the 2021/22 FA Cup final, although it is difficult to equal the 32-year-old in terms of genuine on-field experience. The hirsute 'keeper has made nearly 500 games for Internacional, Roma, and Liverpool since 2013, and has been capped 71 times by Brazil over the last decade, winning almost every trophy available since transferring to Anfield for a world record £67 million in 2018.

In contrast, the 26-year-old Kelleher has only 26 caps for the Republic of Ireland since making his international debut over three years ago, and Alison's status as the Reds' No. 1 has limited him to only 56 appearances in all competitions in five years at the club, nearly half of which came last season.

However, the Brazilian has failed to reach that number in his five completed seasons since, suffering a long-term calf injury just 39 minutes into the Reds' 2019-20 title-winning campaign, which sidelined him for seven league games, before having hip and shoulder difficulties the following season. Meanwhile, earlier this year, Alisson tore his hamstring in training, sidelining him for another seven Premier League games, and he has missed five league games this season due to the same ailment.

In comparison, Kelleher, six years Alisson's junior, has been injury-free since making his Liverpool debut in September 2019, except for a two-month absence due to a hamstring pull at the start of the 2022/23 season.

As a Brazilian, the 32-year-old has always been at ease playing as a sweeper-keeper, and he has three league assists to his name throughout his Reds career, as well as a memorable goal to lead his club to a crucial late win against West Bromwich Albion in May 2021. In recent seasons, however, the odd uncharacteristic error has crept into this aspect of Alisson's game, as seen when he gifted Vinicius Junior an equalizer in a last-16 Champions League clash with Real at Anfield in February 2023, before being at fault for Gabriel Martinelli's goal in February's crucial 3-1 loss to title rivals Arsenal.

In comparison, 32% of Kelleher's touches in Premier League games have come outside his box, though his passing has been just slightly inferior to Alisson's so far this season [72.13% pass completion rate compared to 81.54%], so Liverpool lost nothing in this regard with the Irishman in goal.

Sure, temperament, experience, availability, and ball-playing ability are all important characteristics of the modern goalie, but ultimately, what matters most when evaluating shot-stoppers is their ability to keep the ball out of the goal. In that sense, both players have started six league games this season, with Alisson conceding only two goals to Kelleher's six and keeping three clean sheets, one more than the Republic of Ireland international has done.

In those six games, Alisson faced 17 shots, making 15 saves with an 88.2 save % and saving 2.66 goals, whereas his backup faced 23 shots, making 18 stops with a save percentage of 73.9 and preventing 1.13 goals.
Finally, it is when a save is made during a match that can influence its outcome, as keeping out a goal with the score 1-0 is very different from doing so with the score 3-0, as seen when Kelleher recently made world-class saves to deny Brighton's Georginio Rutter, followed by Aston Villa's Amadou Onana and Diego Carlos, to help Liverpool secure narrow victories.

And if you broaden this metric to all competitions, Kelleher has avoided 4.5 goals in 10 games, compared to Alisson's 2.6 in eight, with neither player making an error that resulted in a goal in any game this season.

Slot has always been clear about his goalkeeping pecking order, stating following Liverpool's victory over Villa before the international break: "Alisson is, and will be, our first goalkeeper if he is fit."

"Caoimhin performed admirably last season and this season as well. But once Alisson is well, he will be our primary goalkeeper. And, despite Kelleher crucially saving Mbappe's spot-kick at Anfield on Wednesday, prompting former Liverpool forward Steve McManaman to say on commentary: "Kelleher's not a No 2, he's a No 1 in equal right," the Reds manager reiterated his stance: "Every decision is difficult, but I believe I have been clear about that. But tonight should be about Caoimhin, who is so important to us.

"It was an important occasion for him. When I saw Mbappe behind the ball, I thought about what everyone else did: 'This will surely go in'. But he rescued it. A special occasion for Caoimhin. That should be the headline, rather than what I stated a few weeks ago.

However, in terms of both the actual eye test - Kelleher impressed greatly when covering for Alisson for extended periods last season and this - and the actual raw data, there is a strong argument to be made that Liverpool would lose nothing by sticking with the Irishman and following the age-old mantra of 'never change a winning team'.