While the home side drop down to this competition after finishing third in their Europa League group, Frankfurt were only able to achieve a runner-up spot behind PAOK in this tournament.
Match preview
Having made some notable additions during the January transfer window, Dino Toppmoller would have been anticipating an immediate improvement from his Frankfurt squad in recent weeks.
Instead, prevailing just once in their last four Bundesliga fixtures has further dampened the mood at Deutsche Bank Park, Frankfurt now five points adrift of fifth position.
At the weekend, they were held to a 1-1 draw at home to Bochum, a result that came on the back of losing 2-0 away at 16th-placed FC Koln.
In total, Toppmoller has delivered just four wins in 13 outings, and there is increasing pressure to deliver in this competition after under-performing in the group stage.
Frankfurt finished a considerable seven points adrift of runaway winners PAOK, losing three times in a group that they would have expected to win in order to avoid a date with the last 32.
Meanwhile, Union SG have dropped down from the Europa League after only finishing third behind Liverpool and Toulouse in their group.
The Belgian outfit could have few complaints having losing to bottom-placed LASK Linz on matchday four, and they have been presented with arguably the toughest test possible in this round.
Alexander Blessin will be frustrated with his team's recent form, a 2-1 reverse at Club Brugge in the cup being followed by a 2-2 draw at home to Westerlo last time out.
Nevertheless, Union SG remain eight points clear at the top of the Belgian top flight, a consequence of a 19-match undefeated streak in the competition.
Union SG form (all competitions):
D
W
D
W
W
D
Eintracht Frankfurt Europa Conference League form:
W
L
W
W
L
L
Eintracht Frankfurt form (all competitions):
W
W
D
W
L
D
Team News
Although Union SG ultimately disappointed last time out, only a 95th-minute penalty cost them all three points against Westerlo, so Blessin may not overly tinker with his starting lineup.
Alessio Castro-Montes may return to the team at right wing-back, but Dennis Eckert Ayensa will likely remain among the replacements as Gustaf Nilsson and Mohammed Amoura lead the line.
Christian Burgess is likely to be recalled to the back three to take the spot of Ross Sykes.
Donny van de Beek moved to Frankfurt on loan with the expectation of featuring in this competition, but the Dutch midfielder has been left out of the squad.
Hugo Larsson may miss out on the left flank due to injury, while could see a start for Hugo Ekitike, while Sasa Kalajdzic's involvement is dependent on the birth of his child.
Union SG possible starting lineup:
Moris; Burgess, Mac Allister, Machida; Castro-Montes, Rasmussen, Vanhoutte, Puertas, Lapoussin; Amoura, Nilsson
Eintracht Frankfurt possible starting lineup:
Grahl; Buta, Koch, Pacho, Max; Skhiri, Gotze; Knauff, Chaibi, Ekitike; Marmoush
We say: Union SG 1-2 Eintracht Frankfurt
Both of these teams will have ambitions to go deep in this competition and we expect one of the most competitive and highest-quality contests of the round. Although Union SG will have the confidence that they can prevail at home, the superior experience is with Frankfurt, who may edge this fixture.