A thrilling first leg saw the Belgian league leaders come back from two goals down to level the tie heading into the return in Germany.
Match preview
Frankfurt boss Dino Toppmoller will have been extremely disappointed in the manner of how his side threw away their 2-0 lead in Brussels last week.
A lightning-fast start saw the 2022 Europa League winners race into a healthy lead after just 10 minutes, with Fares Chaibi and Sasa Kalajdzic netting for the visitors.
However, a gift allowed Union SG back into the game, before Gustaf Nilsson was able to equalise in the second half, with the game finishing 2-2.
An inability to hold onto a lead cost Frankfurt once again in the Bundesliga at the weekend too, as they were pegged back three times in a 3-3 draw with Freiburg on Sunday.
That saw them lose further ground in the race for the top four, despite Borussia Dortmund also drawing, but RB Leipzig moved seven points clear of Toppmoller's men thanks to their win.
There has been a notable drop-off in form on the continent too after Oliver Glasner led them to Europa League glory and the knockouts of the Champions League, as they limped through their UECL group behind PAOK Salonika, losing at Aberdeen on the way.
That could give increased belief to Union SG, who sit eight points clear at the top of the Belgian Pro League, with a league title and a spot in the Champions League next season within reach already.
A 3-1 win away at Kortrijk maintained that lead at the top and also ended a three-game winless run ahead of this trip to Western Germany.
Away form on the continent was not their strong point by any means during the Europa League group stages though, as Alexander Blessin's side failed to score a single goal on the road, losing comfortably to LASK Linz and Liverpool.
Les Unionistes will seek a morale-boosting win in the first of two huge second legs they have on the horizon over the next seven days, as they will also face Club Brugge in the Belgian Cup semi-finals next midweek, trailing 2-1 from the first leg, as they will look to continue to fight on three fronts.
Eintracht Frankfurt Europa Conference League form:
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Eintracht Frankfurt form (all competitions):
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Union SG Europa Conference League form:
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Union SG form (all competitions):
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Team News
Frankfurt forward Kalajdzic received the devastating news that he has torn his ACL once again earlier this week, the third such injury of his career already, which has now ended his season.
The tall Austrian striker joins Mehdi Loune on the sidelines with the same injury, while Hugo Larsson is also absent in midfield with a tendon issue.
Robin Koch started the first leg, but was suspended domestically for their 3-3 draw with Freiburg, so the defender is set to return to the side here.
Likewise on the opposing side, Loic Lapoussin missed Union SG's win at the weekend through suspension, but the Malagasy wide man is also likely to return to the starting XI.
However, a hand injury makes Kevin Mac Allister a doubt for this one, while Charles Vanhoutte will miss out after being sent off late on in the first leg.
Elsewhere in the Union SG ranks, Fedde Leysen (shoulder) and Kevin Rodriguez (muscular) will also be absent for Blessin's side.
Eintracht Frankfurt possible starting lineup:
Trapp; Tuta, Koch, Pacho; Aurelio Buta, Skhiri, Gotze, Nkounkou; Dina Ebimbe, Chaibi, Marmoush
Union SG possible starting lineup:
Lindner; Castro-Montes, Sykes, Burgess, Machida; Rasmussen, Sadiki, Lapoussin; Amoura, Nilsson, Puertas
We say: Eintracht Frankfurt 2-1 Union SG
Frankfurt looked certain to progress within just 10 minutes of the first leg, but a spirited fightback has kept the tie alive, and shows that Toppmoller's side cannot underestimate the visitors despite their limited European experience.