Having already sealed top spot in Group E with four wins from their opening five matches, Jurgen Klopp was afforded the luxury of resting several of his key players ahead of a busy festive schedule.
The Reds’ heavily rotated side provided early encouragement for Union SG, who knew only a victory would be enough to give themselves an outside chance of qualifying for the knockout round.
Noah Sadiki and Koki Machida both wasted half-chances for the hosts in a cagey opening quarter-hour, as Liverpool struggled to gain a foothold in the contest.
Union SG continued to probe for a breakthrough, and their persistence was duly rewarded in the 32nd minute when Mohamed Amoura broke the offside trip to finish beyond a stranded Caoimhin Kelleher.
Despite that setback, Liverpool found a swift response from an unlikely source, as Harvey Elliott’s corner landed at the feet of Jarell Quansah, who took one touch to compose himself before rifling home a superb effort.
The visitors weren’t level for long though, as Les Unionistes restored their lead two minutes before half-time when Cameron Puertas received a neat pass from Amoura and fired low past Kelleher at his near post.
With second-placed Toulouse holding up their end of the bargain against LASK in Group E’s other fixture, Union SG looked determined to press home their advantage in the early stages of the second half.
The hosts thought they’d scored a third shortly after the hour mark, but Puertas’ well-taken strike was correctly disallowed for a handball in the build-up.
Grateful for that reprieve, Liverpool went in pursuit of an equaliser with time ticking into the final 20 minutes, however clear-cut chances came at a premium - despite the introduction of Darwin Nunez.
The hosts came close to adding late gloss to the scoreline, but the one-goal lead proved enough, as they sealed their place in the Conference League play-off round.
As for already-qualified Liverpool, they’ll be keen to bounce back in a crunch Premier League clash against Manchester United on Sunday after a first defeat in seven matches.
Flashscore Man of the Match: Mohamed Amoura (Royale Union SG)
In the other Group E clash, Toulouse knew a win over LASK would see them qualify for the Europa League knockout stages and they did what they needed in winning 2-1 in Linz.
Read all about that match here.
Qarabag seal progression
In Group H, Bayer Leverkusen had secured top spot but Qarabag and Molde were both in play for second place and a spot in the Europe League knock-out phase.
Leverkusen battered Molde 5-1 to effectively end their hopes as Qarabag beat Hacken 2-1 to go through.
Other Europa League results:
Group F
Panathinaikos 1 Maccabi Haifa 2
Group G
