The hosts are languishing in 17th place and could be relegated this weekend, while die Eisernen are hovering just above the bottom three with two games left to play.
Match preview
Last weekend, Koln moved to the brink of relegation after they failed to claim all three points in their goalless home draw against European hopefuls Freiburg.
Having won just one of their last 13 matches, the Billy Goats are now in a perilous position with five points separating them from 16th-placed Mainz and six points keeping them from Union in 15th position.
With only two games left to play, Koln's five-year stay in the Bundesliga appears to be nearing its conclusion, but a win on Saturday would at least send their survival fight to the final day of the season.
However, claiming three points will be easier said than done, especially as Koln have never beaten Union in any of their nine Bundesliga meetings (D2, L7).
As a result, the hosts are looking to claim a head-to-head win for the first time since April 2014 when they recorded a narrow 2-1 victory in the second tier.
Marco Grote has been appointed Union interim boss for the second time this season after the decision was made to dismiss Nenad Bjelica following Sunday's 4-3 defeat to relegation rivals Bochum.
After conceding three goals in the first period, Union offered an improved display following the break to run Bochum close at the Stadion An der Alten Försterei, but the second-half performance could not prevent Bjelica from losing his job after less than six months in charge.
The Croatian won five of his first 10 league matches (D2, L3), but his following 10 Bundesliga games produced just one solitary victory (D3, L6).
Union will be guaranteed to avoid automatic relegation if they pick up at least a point this weekend, but they will still be aiming for all three points due to the slender one-point gap that separates them from Mainz in the relegation playoff spot.
While Koln have scored the fewest goals in the Bundesliga, Union head into the weekend as the second-lowest scorers, and their struggles in front of goal are demonstrated by the fact they have failed to find the net in any of their last four away matches.
FC Koln Bundesliga form:
D
W
L
L
D
D
Union Berlin Bundesliga form:
D
L
L
L
D
L
Team News
The hosts remain without the injured trio of Luca Kilian (knee), Florian Dietz (shoulder), and Davie Selke (metatarsal).
In positive news, Mark Uth, Leart Paqarada and Dejan Ljubicic could be available for selection after returning to training following their respective bouts of illness.
Julian Chabot was forced off against Freiburg with a thigh problem, but Koln boss Timo Schultz is hopeful the defender will be passed fit for Saturday's home clash.
Although Union have reported no fresh injury concerns, they are still unable to call upon Danish goalkeeper Jakob Busk due to a calf injury.
Vertessen could be recalled to the starting lineup after he came off the bench against Bochum to net his third goal since arriving from PSV Eindhoven in January.
Leeds United loanee Brenden Aaronson dropped out of the side last weekend, but he could be brought back into the fold after starting six consecutive games prior to the recent 4-3 defeat against die Unabsteigbaren.
FC Koln possible starting lineup:
Schwabe; Schmitz, Hubers, Chabot, Finkgrafe; Huseinbasic, Martel; Thielmann, Waldschmidt, Kainz; Alidou
Union Berlin possible starting lineup:
Ronnow; Doekhi, Vogt, Leite; Juranovic, Aaronson, Khedira, Tousart, Gosens; Hollerbach, Vertessen