The visitors, meanwhile, returned to the Bundesliga's top four positions with a 3-1 win against Hoffenheim at the weekend.
Match preview
Having punched considerably above their weight following their promotion from Germany's second tier last season, Werder Bremen came crashing back down to Earth on Saturday in their first game since the World Cup.
Five goals in the opening 36 minutes saw Bremen fall 5-0 behind in Cologne, with Niclas Fullkrug's 11th league strike of the season representing the visitors' only moment of celebration during a miserable afternoon.
The nature of the 7-1 scoreline was made all the more surprising by the fact that Koln still remain one point behind Bremen in the table, and had headed into the encounter off the back of three straight defeats. Instead, it is now Ole Werner's side who are on that exact sequence of losses.
With only six points separating them and Stuttgart in the relegation playoff position, it is far from an ideal time for Werner's side to be facing Union Berlin, who are likely to be full of confidence when they arrive in Bremen on Wednesday.
Indeed, after failing to win any of their last three league games prior to the World Cup, Saturday's 3-1 victory against Hoffenheim was a much needed one for Urs Fischer and his players.
In typical fashion under Fischer's tutelage, they showed great character to comeback from 1-0 down at half time after Ihlas Bebou's 43rd-minute strike, with two headed goals from defender Danilho Doekhi and an injury-time effort from Jamie Leweling providing their side with all three points.
Having missed out on qualifying for the Champions League for the first time in the club's history by a solitary point last season, the 56-year-old is determined not to fall short once again as he continues to take Die Eisernen to greater heights than ever before.
Although finishing in the Bundesliga's top four positions represents their best chance of achieving this tremendous feat, Union Berlin are still fighting on three fronts, with a two-legged tie against Ajax awaiting them in the last 32 of the Europa League next month after facing Wolfsburg at home in the last 16 of the DFB-Pokal.
In the meantime, though, Fischer will be aiming to collect maximum points from league fixtures against Bremen, Hertha Berlin and Mainz 05 ahead of a potentially- pivotal clash against RB Leipzig next month.
Werder Bremen Bundesliga form:
L
W
W
L
L
L
Union Berlin Bundesliga form:
L
W
L
D
L
W
Union Berlin form (all competitions):
W
W
L
D
L
W
Team News
Bremen will remain without Romano Schmid and Felix Agu due to knee and tendon issues respectively, with the duo both unlikely to return until towards the end of next month.
Having been thrashed 7-1 at the weekend, it would be somewhat surprising if Werner did not make some changes to his starting XI, with Amos Pieper a near certainty to return to the back three after making his comeback from an ankle injury during the second half.
Union Berlin, meanwhile, will travel without Morten Thorsby and Andras Schafer due to abdominal and foot injuries, while new signing Jerome Roussillon is unlikely to be involved due to a lack of match fitness.
Sven Michel should be available to be named among the substitutes after missing his side's win against Hoffenheim on Saturday through illness, while Diogo Leite could return to the back three after serving his one-match suspension.
Werder Bremen possible starting lineup:
Pavlenka; Pieper, Veljkovic, Friedl; Weiser, Gross, Stage, Gruev, Buchanan; Fullkrug, Ducksch
Union Berlin possible starting lineup:
Ronnow; Leite, Knoche, Doekhi; Trimmel, Haraguchi, Khedira, Haberer, Griesselmann; Becker, Siebatcheu
We say: Werder Bremen 1-2 Union Berlin
Werder Bremen looked wide open defensively against Koln during their 7-1 defeat on Saturday, with only VfL Bochum and Schalke 04 possessing inferior goals against tallies than them so far this season.
While Werner will have worked studiously on the training round in an attempt to tighten things up, these things rarely happen overnight, and we can envisage Union Berlin causing plenty of problems in transition on Wednesday to record successive victories.