One or both of the sides will finish for the break in the bottom three, depending on the result here, and the results of games involving Darmstadt and Mainz.
Match preview
Union and Koln come into the game level on points after both suffering disappointing away defeats at the weekend.
The two are on 10 points apiece, with Darmstadt and Mainz both below them in the automatic relegation spots a point further back.
The only thing keeping Union out of the relegation playoff place is the fact they have scored more goals, but they were unable to find the net in a 3-0 defeat to Bochum at the weekend.
Last season's Champions League qualifiers still have a game in hand and look most likely to be able to play their way out of trouble given the quality in their ranks, but that was a huge setback for new manager Nenad Bjelica.
The Croatian's first match in charge was a disappointing draw with 10-man Braga, but they followed that up with a win over Borussia Monchengladbach, and a courageous defeat to Real Madrid.
Spirits do appear to be higher though compared to when they went on a 16-game winless streak, in which time they lost 12 in a row, as they have since lifted themselves off the bottom and out of the drop zone.
They also now face a side they have a superb record against, as Union are unbeaten in 10 meetings since 2019, winning seven, but they did only draw 0-0 here in March.
Another low-scoring affair could be on the cards, as Koln have scored a meager 10 goals in the Bundesliga after 15 games this season - the lowest in the division.
Steffen Baumgart has been able to make his side harder to break down, but that has come at a cost in an attacking sense.
Koln have lost only three of their last eight games, but there have been four draws in that run too, with the solitary win coming against bottom-of-the-table Darmstadt.
That success took them off the bottom, and a draw with Mainz two weeks ago kept them out of the automatic relegation spots, but they were beaten at Freiburg on Sunday.
It was an even game until Julian Chabot's 62nd-minute dismissal, after which Freiburg were dominant, scoring their two goals and registering an xG of 4.28.
Another blank extended a run of just three goals in their last seven games, but two clean sheets at the start of December did contribute to four important points.
Union Berlin Bundesliga form:
L
L
L
D
W
L
Union Berlin form (all competitions):
L
D
D
W
L
L
FC Koln Bundesliga form:
D
D
L
W
D
L
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Team News
Union left wing-back Robin Gosens is a doubt for this one with a knock, so Jerome Roussillon could start again in that position here.
On the opposite side, Christopher Trimmel will compete for a start with Josip Juranovic, while in the centre of defence, despite Danilho Doekhi's absence, Leonardo Bonucci is still yet to feature under Bjelica.
In midfield, Andras Schafer has been playing himself back into fitness after a long layoff, while Aissa Laidouni appears to have lost his place in the side under the new manager.
Chabot's red card at the weekend will see him suspended for this one, so Luca Kilian could come into the Koln back four to replace him.
Apart from that, the visitors could be largely unchanged, with Leart Paqarada, Faride Alidou and Sargis Adamyan still the only players absent.
Benno Schmitz came in for Rasmus Carstensen at the weekend, but the Genk loanee may reclaim his place at right-back for this one.
Union Berlin possible starting lineup:
Ronnow; Juranovic, Knoche, Diogo Leite, Roussillon; Khedira, Haberer; Hollerbach, Volland, Becker; Behrens
FC Koln possible starting lineup:
Schwabe; Carstensen, Hubers, Kilian, Heintz; Martel, Ljubicic; Thielmann, Kainz, Maina; Selke
We say: Union Berlin 1-0 FC Koln
Union have shown more promising signs under Bjelica, and they will want to quickly get over their woeful result at the weekend against a side they have a good recent record over.