A youthful Liverpool outfit ended their Europa League Group E campaign on a sour note as Union SG overcame Jurgen Klopp's team 2-1 at the Constant Vanden Stock Stadium.
Having assured themselves of a first-placed finish on the penultimate matchday, the Reds were only playing for pride in Belgium, where Mohamed Amoura and Cameron Puertas's efforts either side of a Jarell Quansah response propelled Union SG to a thoroughly deserved success.
The hosts were still in with a shot of sneaking into second place ahead of Toulouse, but with the French side overcoming LASK Linz 2-1 in the group's other game, Les Unionistes will settle for a place in the Europa Conference League knockout round playoffs.
With their tickets to the last 16 already in the bag, Liverpool put out an experimental XI comprising fringe and youth players in abundance, namely Kaide Gordon, Luke Chambers, Conor Bradley and Ben Doak.
Given the distinct lack of experience in the visitors' XI, Klopp's side were unsurprisingly on the back foot early doors, as Noah Sadiki flashed a shot wide of Caoimhin Kelleher's goal with just 10 minutes gone.
The Reds goalkeeper was then called into action to parry Amoura's strike away in the 17th minute before also making himself big to deny Gustaf Nilsson one-on-one four minutes later, although the offside flag would have come to Liverpool's rescue anyway.
However, there would be no such reprieve for Klopp's side in the 32nd minute, as Amoura got in behind the Liverpool high line and sprinted into the area, where Kelleher sought to charge him down.
The Irish goalkeeper dived at Amoura's feet and got a hand on the ball, but the latter was the beneficiary of a lucky bounce off his shin, allowing him to crash home into an open goal.
Les Unionistes' advantage was wiped out only seven minutes later via an unlikely source, though, as Quansah controlled a Curtis Jones corner expertly and rifled in the equaliser from around 12 yards to open his Liverpool account.
Five weeks on from being denied his first Reds goal against Toulouse, though, Quansah's moment was ruined once again, as with 43 minutes on the board, the vibrant Amoura fizzed a slick pass into the centre for Puertas, who caught Kelleher out at his near post to restore Union SG's lead.
Alexander Blessin's charges picked up where they left off after the restart and had the ball in the back of the net again via a sublime first-time Puertas strike in the 62nd minute, but play was brought back for a Lazare Amani handball, and the goal was disallowed following a quick VAR check.
A couple more opportunities went begging for Union SG, as Nilsson headed wide in the 70th minute before Kelleher thwarted Amoura again two moments later, but Liverpool were not going down waving the white flag, as Ryan Gravenberch shot straight at Anthony Moris from range with 11 minutes remaining.
Kelleher even came up for a last-minute corner as Liverpool huffed and puffed for a leveller, but Union SG survived one final Reds set-piece to ensure that their continental journey would continue.
Klopp's men now have a blockbuster Premier League tie with Manchester United on the menu for Sunday, while Union SG host Mechelen in their next Jupiler Pro League clash.
