The two sides will butt heads at the Constant Vanden Stock Stadium in Group E, where Liverpool and LASK Linz are also vying for the sole qualification spot straight into the last 16.
Match preview
Hitherto a dominant force of Belgian football during the early 20th century - collecting 11 top-division titles between 1904 and 1935 - Union SG are starting to announce themselves on the European stage once again, progressing to the last eight of the 2022-23 Europa League before succumbing to Bayer Leverkusen's supremacy.
Amid their deep run on the continent, Union 60 came within a single point of ending their 88-year Belgian title drought but were forced to settle for the bronze medal behind champions Royal Antwerp and second-placed Genk, who only bested Union SG owing to their superior regular-season points total.
A third-placed finish saw Union SG enter their second Europa League campaign in the qualifying round playoffs, where a 3-0 aggregate win over Lugano propelled Alexander Blessin's men into the group phase with relative ease, but their domestic form has suffered amid their latest continental quest.
Indeed, Union SG come into Thursday's match winless in three Jupiler Pro League matches - suffering a 4-0 thrashing away to Mechelen, drawing 2-2 with holders Antwerp before the international break and most recently going down 2-0 at home to Genk on Saturday.
Of course, those results should not be taken fully at face value given that Union SG faced last year's Belgian champions and runners-up, but Blessin's men will not have the full home advantage in their favour this week, as the tie will take place at Anderlecht's headquarters due to the Union stadium not meeting UEFA requirements.
While Union SG were competing at the top end of the Belgian table last term, Toulouse accepted mid-table mediocrity in their first season back in Ligue 1, placing 13th in the top flight to complement their magnificent Coupe de France exploits.
Locking horns with defending champions Nantes in the showpiece match, Toulouse snatched the trophy out of Les Canaris' grasp with an emphatic 5-1 victory at the Stade de France to win their first-ever major honour in their 86-year history, not counting a trio of Ligue 2 title triumphs.
Success in France's premier cup tournament booked Toulouse's spot in the Europa League group stage, but for a short while, there were fears that Les Violets could be barred entry into the tournament owing to UEFA regulations; Toulouse's owners RedBird Capital Partners are also in charge of AC Milan, and no two teams with the same shareholders can participate in the same competition.
However, with the Rossoneri sealing Champions League qualification, Toulouse were given the green light to enter the group-stage draw for the second time, having also failed to make it to the knockout stages during their tournament debut in the 2009-10 campaign.
Coupe de France-winning coach Philippe Montanier has since handed control of the reins over to Carles Martinez, but his side have made an indifferent start to the Ligue 1 season, failing to win any of their last four matches since teaching Nantes another footballing lesson in a 2-1 triumph on the first matchday.
Union SG Europa League form:
W
W
Union SG form (all competitions):
W
L
W
W
D
L
Toulouse form (all competitions):
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Team News
Union SG's home defeat to Genk at the weekend was painful for more than one reason, as 18-year-old midfielder Noah Sadiki was withdrawn with an unspecified injury 11 minutes before full time, but updates on his condition have been scarce.
As a result, Sadiki could very well make himself available for the visit of Toulouse, although Mathias Rasmussen is waiting in the wings, and Blessin is not believed to be working around any other fitness concerns for his side's European opener.
Before reuniting with his brother - Liverpool new boy Alexis Mac Allister - later on in the group stage, Kevin Mac Allister will try to stifle the Toulouse attack as part of the hosts' three-man backline.
Meanwhile, Toulouse coach Martinez has a trio of absentees to contend with for the opening matchday, including attacking duo Naatan Skytta and Ibrahim Cissoko, who are dealing with tibia and metatarsal fractures respectively, while left-back Oliver Zanden is recovering from hip surgery.
A plethora of Toulouse's cup winners departed the club in the summer, including Lens-bound midfield fulcrum Stijn Spierings, but the Dutchman has been loaned straight back to Les Violets after struggling to acclimatise to life at the Stade Bollaert-Delelis.
One man who has benefitted from the old guard leaving is 18-year-old goalkeeper Guillaume Restes, who should start here after helping his side claim a point in a goalless draw with Marseille at the weekend, making six saves at the Orange Velodrome.
Union SG possible starting lineup:
Moris; Mac Allister, Leysen, Burgess; Castro-Montes, Rasmussen, Vanhoutte, Lapoussin; Puertas, Amani; Eckert Ayensa
Toulouse possible starting lineup:
Restes; Desler, Costa, Nicolaisen, Suazo; Schmidt, Spierings, Sierro; Aboukhlal, Dallinga, Genreau
We say: Union SG 1-1 Toulouse
Neither Union SG nor Toulouse are experiencing a purple patch of domestic form at the moment, but if last year is anything to go by, the 11-time Belgian champions are not to be underestimated on the European stage.