With no Premier League games to his name so far and just four domestic cup appearances under his belt, appearing in the most glamorous and elite competition in the world could have been a little daunting.
But Kelleher was calm throughout, showed several of his best attributes and even produced a couple of crucial saves, including a last-minute stop from Klass-Jan Huntelaar to preserve Liverpool’s win and his own clean sheet.
It’s all the more remarkable considering the 22-year-old wasn’t even a goalkeeper until well into his teenage years.
Up until the age of 14, Kelleher played as a striker in Irish youth football, which perhaps gives hints as to why he’s so comfortable in possession of the ball.
Speaking to the Irish Times, his former coach at Ringmahon Rangers U14s, Stephen Bermingham, explained that many of the traits on show against Ajax are those which have been Kelleher’s hallmarks from the moment he put on a pair of gloves.
“His attitude, his commitment, determination, athleticism, everything – you could see his calmness against Ajax, he was like that from the minute he became a goalkeeper,” he said.
“Because he was a very good outfielder, he has that strong distribution. That was a great starting point and he was just so calm and relaxed. He always took in the information – you’d say something to him once and he’d get it.
“He has worked extremely hard at goalkeeping since he was 14, to improve all aspects of his game.”
Those comments and variations of them have been mirrored several times over the last 18 months; the likes of Andy Lonergan, Pepijn Lijnders and Alisson Becker, all individuals who have seen him up close on a regular basis, speak in glowing terms of Kelleher’s ability to learn, his composure and his all-round quality.
Within the Irish national team set-up there is also a feeling that he’d be good enough to be playing as first-choice right now – if he was doing so at club level.
That won’t happen just yet at Liverpool, but if he plays against Wolves in the Premier League at the weekend with Alisson still sidelined by injury then there’s a good chance a few clubs will start to come looking for a loan deal sooner rather than later.
Back at age 14 after a conversation between coaches and parents, it was less than a year between Kelleher switching positions and being called into the Irish international development squad, before the young ‘keeper then went on to have trials with Aston Villa and be recommended to Manchester City.
They passed him up and the Reds quickly moved in; Kelleher signed in 2015 and has spent the next half-decade honing his skills while moving up through the Academy, before finally getting his first chances under Klopp.
That means it was just three years between switching to a completely new footballing role and being signed up by one of the biggest clubs in the world, another indication of how quickly he learns.
Sean Fitzgerald, club secretary of Ringmahon, notes a few reasons why Kelleher has made a successful transition.
“Because he was a striker himself, he was able to anticipate what a striker was going to do,” he said.
“He had no fear, either – a ball would bounce in the six-yard box and he’d pounce on it. You just knew straight away that he was one of these naturally gifted fellas.”
With a league debut likely at the weekend, Kelleher will reach half a dozen games for the Reds at first-team level. By common consent, all he is lacking is senior experience at this point, and that can only be earned one game at a time.
All Liverpool fans want Alisson Becker back in place as he’s one of the greatest in the world, but a safe pair of hands and some competent footwork from his No. 2 is perhaps one of the few things the squad has been missing for some time and Kelleher now looks capable of offering both.