The Italian side are struggling in Serie A and have won only once in Europe, while their hosts have picked up just one point from three games so far.
Match preview
When Roma recently earned a first Europa League win of the season, edging out Dynamo Kyiv at Stadio Olimpico as Artem Dovbyk struck the winner against his fellow Ukrainians, prospects of Ivan Juric keeping his job improved.
After a rocky start in Rome, where fans have been protesting against the board and lamenting the loss of club legend Daniele De Rossi - who occupied the dugout before Juric - it seemed that the Croatian coach was on borrowed time.
That win, following an opening draw with Athletic Bilbao and then defeat in Sweden to Elfsborg, lifted Roma up to 19th in the Europa League table, but they have since slid down to 12th in Serie A.
The Giallorossi have lost three of their last four league games following Sunday's mistake-strewn 3-2 defeat at Hellas Verona - formerly managed by Juric - including a 5-1 thumping by top-six rivals Fiorentina.
Their new boss has recorded just four wins in 10 matches, leading to speculation that club owners, the Friedkin Group, would re-hire one of his predecessors, such as De Rossi or Claudio Ranieri - the latter of whom retired from management when leaving Cagliari in May.
Against that background, Roma must head to Brussels, where history will at least be on their side: they are undefeated in 14 meetings with Belgian teams since the mid-1970s.
All four previous meetings between Union Saint-Gilloise and Roma predate the Giallorossi's last loss to opponents from Belgium: the clubs met in the old Inter-Cities Fairs Cup in both 1959 and 1960 - either side won once, with two draws.
Union will prepare for this week's encounter in even worse shape than their Italian counterparts, as a 1-0 loss away to Midtjylland last month left them occupying 29th place in the Europa League standings - trailing the playoff places by two points.
Despite having 16 more shots than their opponents so far, Sebastien Pocognoli's side are winless from three league-phase matches, having also been beaten by Fenerbahce and drawn with Bodo/Glimt since being knocked out of Champions League qualifying.
Pocognoli may have begun his reign by lifting the Belgian Super Cup, but Union are treading water in the Belgian Pro League this season, sitting ninth following a draw and a defeat in their two most recent outings.
Les Unionistes did have a rare taste of success when beating Eupen in the Belgian Cup at the end of October, but that was their sole win from six games last month.
Union SG Europa League form:
L
D
L
Union SG form (all competitions):
D
D
D
L
L
D
Roma Europa League form:
D
L
W
Roma form (all competitions):
D
L
W
L
W
L
Team News
After a miserable defeat last weekend - and with half an eye on Sunday's league game against Bologna - Ivan Juric is set to make several changes to his Roma XI in Belgium.
Top scorer Artem Dovbyk should be rested, with Eldor Shomurodov handed an opportunity up front, while Stephan El Shaarawy could replace Nicola Zalewski on the left wing. Furthermore, French pair Manu Kone and Enzo Le Fee may make way for Bryan Cristante and rising star Niccolo Pisilli.
Mario Hermoso's recent injury might improve the chances of Mats Hummels finally getting a first start; the visitors will also be without Alexis Saelemaekers, who is still sidelined by an ankle problem.
Meanwhile, Union SG have turned to ex-Everton forward Kevin Mirallas to improve their fortunes in the final third, and the club's new striker coach will take Madagascar international Loic Lapoussin under his wing.
The latter was recently brought back into the fold following his exile with the 'B' team for indiscipline, and he should start in support of Franjo Ivanovic and Kevin Rodriguez up front; David Promise gained confidence from scoring twice in the cup tie against Eupen and is another alternative.
Alessio Castro-Montes and Sofiane Boufal are ruled out, but English defender Ross Sykes - who recently signed a new contract - has returned from a foot injuy.
Union SG possible starting lineup:
Moris; Sykes, Burgess, Mac Allister; Niang, Amani, Sadiki, Vanhoutte, Lapoussin; Rodriguez, Ivanovic
Roma possible starting lineup:
Svilar; Mancini, Hummels, Angelino; Abdulhamid, Paredes, Cristante, El Shaarawy; Pisilli, Baldanzi; Shomurodov
We say: Union SG 0-1 Roma
Roma have completed the most passes and are averaging the most possession (66%) in the Europa League this season but have failed to make such supremacy count.