A first Rangers goal since October 2023 for the irrepressible Nico Raskin, coupled with Vaclav Cerny’s second half strike, proved the difference as Philippe Clement’s men will now avoid a two-legged play-off tie next month.
It was just reward for an impressive league phase campaign for the Gers as they look to embark on another special journey in the competition.
The Gers made two changes to the side from the weekend triumph at Tannadice, with Vaclav Cerny returning from injury and Academy graduate Leon King replacing the ineligible Clinton Nsiala.
Injury struck, however, for King just 13 minutes into the contest following a challenge on the byline that meant Clement was forced into an early change whereby Ross McCausland slotted in at right full-back.
Rangers certainly weren’t perturbed by the early shuffling of the pack as Cyriel Dessers, a man enjoying a rich vein of form, nodded wide from a dangerous James Tavernier free-kick delivery.
The Belgian visitors were showing vulnerability from dead ball situations and they were ruthlessly punished by the Gers on 20 minutes to break the deadlock.
A short corner caught the away side on their heels as Tavernier’s deep delivery found Nico Raskin in acres of space at the far post to nod confidently into the bottom corner.
For all that USG had shown very little attacking endeavour in the opening half-hour, they had the ball in the net when Franjo Ivanovic stabbed home from close-range.
However, Rangers’ blushes were spared when it was ruled by VAR that that the goalscorer had strayed into an offside position.
That proved the cue for Dessers to go close twice to doubling his side’s advantage before the half-time interval.
The Nigerian initially went close from inside the penalty area when he muscled his way through a pack of USG challenges, but a vital last-ditch deflection sent his effort spinning out for a corner.
He then had the ball in the back of the net less than 60 seconds later from the second phase of a corner-kick, but, much like the visitors previously in the half, forward’s instinct saw him stray a centimetre offside.
The visitors emerged for the second period with a renewed intent, going close when Tavernier was forced into a fantastic last-ditch sliding challenge from Mohammed Fuseini after the forward had rounded Jack Butland inside the danger area.
Ivanovic would lift the rebound over, but it was a reminder to Rangers that the job was far from finished in Govan.
However, the Light Blues strengthened their grip on the contest five minutes later and it was the in-form Dessers who crafted the goal.
The Nigerian was sprung down the left-hand side with no-one in support in royal blue, yet he showed tremendous footwork to drive to the byline and wait for Cerny to support inside the penalty area who duly swept home from the resulting pull-back.
A third goal of the evening would surely put the game to bed and it very nearly arrived moments later when Cerny picked up possession 25 yards from goal and watched his right-footed effort crash off the outside of the post.
The hour-mark ushered in the return of John Souttar from injury who took to the pitch for the first time since he was forced off inside the first-half at home to Spurs in this competition in December.
USG were in desperation mode as they sought an avenue back into the game, allowing Rangers to pick their moments to attack into the vacated space.
Hamza Igamane was the beneficiary as he was released into a pocket of space inside the visitors’ half, but the Moroccan lacked the required finish to kill off the game as a contest.
There was a desperate checking of phones around Ibrox as the Gers support crossed their fingers for the sequence of results which would propel the side into the top eight.
In the meantime, Clement turned to his substitutes bench and rewarded 18-year-old Paul Nsio with his European debut alongside a second successive substitute appearance for Zak Lovelace.
Any hopes of a serene conclusion to the contest were dashed with six minutes remaining when Kevin Mac Allister reduced the arrears for USG with a looping header to set up a grandstand finish.
The Belgian side knew an equaliser would throw the cat among the pigeons and they went close to a leveller in stoppage time, but the Gers successfully clung on and, coupled with results elsewhere, celebrated a dramatic top eight finish.
RANGERS: Butland, King (McCausland, 13), Tavernier, Propper, Jefte, Raskin, Diomande, Igamane (Lovelace, 79), Cerny (Souttar, 60), Bajrami (Nsio, 79), Dessers
Subs not used: Kelly, Munn, Lawrence, Rice, Curtis, Hutton
UNION SAINT-GILLOISE: Moris, Mac Allister (Rasmussen, 90), Van De Perre (Ait El Hadj, 29), Ivanovic, Niang, Vanhoutte, Khalaili, Sykes, Sadiki, Leysen, Fuseini
Subs not used: Chambaere, Imbrechts, Rodriguez, Asri, Berradi