Urs Fischer's side now find themselves three points behind Bayern and Borussia Dortmund, while Koln sit 12th - 17 points behind Union - following back-to-back losses.
Match preview
Having kept pace with Bayern Munich throughout the campaign, Union Berlin were expected to give the perennial champions a run for their money last weekend, but Fischer's men barely laid a glove on Bayern.
Following first-half goals from Eric Choupo-Moting, Kingsley Coman and Jamal Musiala, the Iron Ones found themselves 3-0 down at the break, and they were unable to respond in the second half.
In fact, Union failed to register a single shot on target at the Allianz Arena, and the visitors finished the game with just 29% possession, highlighting the gulf between the two sides on the day.
Still, the surprise package from the capital remain firmly in the title race after 22 games, having recorded 13 wins, four draws and five losses this season, while only Bayern have a better defensive record than Union Berlin.
After suffering a solitary defeat in their previous 10 outings across all competitions - picking up seven wins and two draws along the way - Union will be confident of bouncing straight back in front of their passionate fans.
Meanwhile, FC Koln suffered a 2-0 defeat to Wolfsburg last weekend, despite racking up 13 shots while enjoying the lion's share of the ball on home soil.
Steffen Baumgart's side have now lost each of their last two games, with a disappointing 3-0 defeat to relegation-threatened Wolfsburg preceding last weekend's loss against the Wolves.
To make matters worse, Koln have only managed to win one of their last six Bundesliga encounters, with two victories, four draws and six losses coming in their previous 12 league matches.
It is worth remembering that Koln thrashed Werder Bremen 7-1 in January before recording draws against Bayern Munich and RB Leipzig, while they also claimed an impressive 3-0 victory over Eintracht Frankfurt last month.
Still, the Billy Goats remain just seven points outside the bottom three following back-to-back defeats, meaning that Baumgart's men could find themselves in a relegation battle if they don't discover some consistency.
Union Berlin Bundesliga form:
W
W
W
W
D
L
Union Berlin form (all competitions):
W
W
D
D
W
L
FC Koln Bundesliga form:
D
D
D
W
L
L
Team News
Union Berlin midfielder Andras Schafer has missed the past few games due to a foot injury and looks set to be sidelined for the foreseeable future.
That said, the hosts have reported no fresh injury problems ahead of Saturday's clash, meaning that Fischer may be tempted to shuffle his pack after last weekend's loss to Bayern, given the number of options at his disposal.
Sheraldo Becker and Kevin Behrens have featured in all 22 Bundesliga outings for Union this season, while Robin Knoche and Rani Khedira have both made 21 league appearances, so the reliable quartet should start for the hosts.
As for the visitors, they have one of the biggest injury lists in the league at the moment, with Florian Dietz (ACL) and Mark Uth (groin) set to miss the remainder of the campaign.
Jan Thielmann, Kristian Pedersen and Luca Kilian are all suffering from muscle injuries, while Sebastian Andersson is expected to be out until late March with a knee problem.
On top of that, Denis Huseinbasic, Florian Kainz and Benno Schmitz all missed the previous game due to illness, so it remains to be seen whether the trio will be fit enough to face Union Berlin.
Union Berlin possible starting lineup:
Ronnow; Doekhi, Knoche, Leite; Trimmel, Laidouni, Khedira, Thorsby, Roussillon; Becker, Behrens
FC Koln possible starting lineup:
Schwabe; Schindler, Hubers, Chabot, Hector; Ljubicic, Olesen, Skhiri, Maina; Selke, Tigges
We say: Union Berlin 2-0 FC Koln
In addition to suffering two straight losses, FC Koln have failed to score in four of their previous five outings, putting Union Berlin in pole position to claim all three points on Saturday.