The average age of Liverpool's starting XI against Midtjylland was just 24 years and 26 days on Wednesday. It was their youngest line-up in the Champions League.
With top spot in the group and qualification for the knockouts already secured, boss Jurgen Klopp had the luxury of resting some of his regulars and hand out opportunities to some rising stars.
The 1-1 draw turned out to be a good experience for the players involved and here we assess how some of the newer faces fared...
Liverpool's 22-year-old goalkeeper Kelleher had gone into this one on the back of consecutive clean sheets against Ajax and Wolves. There was no shutout this time - but still some encouraging moments.
A goal-line clearance from Fabinho was crucial on 32 minutes and Kelleher saved well at his near post just before half-time as Midtjylland grew in confidence. That was his only save of the opening 45 minutes but the hosts put on the pressure after the break.
It was Kelleher who gave up the penalty, clattering into Anders Dreyer after he broke through on goal. The young 'keeper had little option, in truth, and escaped with only a booking under the double jeopardy rule - but he could not keep out Alexander Scholz's impressive spot-kick.
Kelleher remained sharp despite the setback, pushing away Erik Sviatchenko's header firmly, before punching with authority to ease the strain as Midtjylland looked for a late winner.
Williams has become Klopp's go-to Champions League centre back - this was his sixth appearance in the competition - but the young defender showed on Wednesday that the importance of having an experienced figure alongside him cannot be underestimated.
The 19-year-old came on after 30 minutes against this opposition at the end of October when Fabinho was injured but played alongside him on Wednesday night during the first half, when the pair kept Midtjylland at bay.
However, Klopp replaced Fabinho with debutant Billy Koumetio at the break and there was a clear sense of nervousness at the heart of Liverpool's backline, with the two centre-backs playing in front of young 'keeper Kelleher.
While Williams won the physical battle with Sory Kaba in the first half, the hosts' striker had more joy after the break, and it was Williams who ultimately played Dreyer onside in the lead-in to Midtjylland's penalty.
But Williams stood up well to the challenge, topping the charts for clearances, aerial duels, and tackles (jointly) and helped Liverpool come through the test with a point.
Leighton Clarkson made his senior bow for Liverpool almost exactly a year ago in the Carabao Cup but his Champions League debut took it up a level and the 19-year-old from Blackburn will have learnt plenty from the occasion.
Clarkson sat deepest in the middle three for much of the game but that did not hold him back from joining in with attacks and a nice pass to Takumi Minamino led to a good Jota chance in the first half. He was also aware enough positionally to drop into the right-back slot when Trent Alexander-Arnold had pushed on, from where he pinged an inch-perfect ball forwards into the box for Jota.
The 2018/19 FA Youth Cup winner showed his youthful enthusiasm in the second half with his pressing and harrying of the opposition - he gained possession back more times than any Liverpool player - but was caught out for the penalty after shuffling to left-back when Konstantinos Tsimikas was off the pitch injured.
Clarkson allowed his man Dreyer to run off him and through on goal, where he was fouled by Kelleher, revealing the young midfielder's inexperience in that position. That aside, though, this was a promising step up.
Billy 'The Kid', as Klopp calls him, is highly rated at Liverpool. The teenage centre-back was training with the first team in pre-season and would have made his senior debut sooner if it had not been for some ill-timed injuries.
At half-time the Lyon-born defender was thrown into the action in place of Fabinho, becoming Liverpool's youngest-ever Champions League player at the age of 18 and 25 days.
Like Williams alongside him, Koumetio is tall but a miscued header just four minutes into the second half allowed Midtjyjlland to hit the bar. Not long after, Kaba beat him to the ball to head wide as Midtjylland targeted the new arrival.
There were further nervy moments, including a misplaced pass back to Kelleher which led to a corner, but Koumetio showed glimpses of his talent, making a superb block on Kaba in the build-up to Scholz's offside strike.
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Klopp's verdict on Clarkson, Koumetio's Champions League debuts
Liverpool boss Jurgen Klopp:
"They will [learn from the experience] and especially if we let them make the experience without now making this one game too big. It's a wonderful, wonderful thing to have your first Champions League game but a career is all about how many you will have or how many games overall you will play.
"So both boys have a lot to learn. They are already really skilled but they have a lot to learn and we will give them the time to do so. Again, tonight they helped us a lot because we could rest other players which was very important and there was actually no alternative.
"So yeah, I'm pretty sure it was a big night for both of them and I am really thankful that they are already as good as they are. That's the reason why we can use them, but I know there is a lot more to come."