Meanwhile, Enrico Maassen's Augsburg are scavenging to stave off a relegation fight as they look for their first win in seven games.
Match preview
Last Saturday's 5-0 pummelling at the hands of relegation-battling Bayer Leverkusen must have come like a bolt from the blue for many Bundesliga fans.
Having found themselves inexplicably atop the table for much of the campaign, outside viewers would be forgiven for thinking head coach Urs Fischer was simply an alchemist, turning Union's minnow ironmen into title-contending gold.
In reality, Fischer must have seen the result against Leverkusen as a stark wakeup call for his team, a reminder of the gargantuan challenge they are up against to stay so high up the Bundesliga this season.
The financial disparity between Union and the clubs that surround them is stark, and the challenge to both maintain excellent league form whilst competing in the Europa League - a tournament notorious for grinding down teams throughout a season - is one new to Die Eisernen. As this brutal fixture list wears on, it is only natural that Union's vulnerabilities will begin to emerge.
Indeed, Fischer was compelled to make several changes after their pivotal Europa League victory against Union Saint-Gilloise; absent was the defensive presence of Dutch wunderkind Danilho Doekhi, arguably Die Eisernen's best player this season.
Added to Fischer's woes was the loss of Danish goalkeeper Frederik Ronnow, whose performances this season must surely put him in contention to replace Kasper Schmeichel as Denmark's first-choice goalkeeper. The jury is out if the young Leverkusen loanee Lennart Grill can live up to the standards set by Ronnow.
Perhaps it will be a comfort to Union that they will be facing a struggling Augsburg team on Wednesday; Maassen's Fuggerstadters have picked up just two points in their last six matches, while their 14 points have them hovering above the relegation zone in 13th place.
That said, it has not all been doom and gloom over at the WWK Arena - Maassen's three victories in a row in September included a titanic victory against Julian Nagelsmann's Bayern Munich, and away wins against Werder Bremen and Schalke 04.
Alongside the shot-stopping success of the Polish keeper Rafal Gikiewicz, Augsburg's fluid attacking contingent of Mergim Berisha, Ermedin Demirovic, Fredrik Jensen and Florian Niederlechner have managed to score in all but two of their fixtures this season.
Union will have to be wary of Augsburg's potential in counter-attack after their aggression and inability to adapt to Leverkusen's attacking overloads cost them at the weekend, while individual errors from Grill and Rani Khedira only added to their tactical misplay.
Augsburg will therefore look to move the ball vertically in transition, and exploit any misplaced aggression from Fischer's Eisernen.
Union Berlin Bundesliga form:
L
W
W
L
W
L
Union Berlin form (all competitions):
W
L
W
W
W
L
Augsburg Bundesliga form:
W
D
L
D
L
L
Augsburg form (all competitions):
D
L
L
D
L
L
Team News
Union Berlin will be without goalkeeper Frederik Ronnow for Wednesday with a "structural muscle injury" after the Dane missed out the second half of the Leverkusen match, and loanee Lennart Grill, who came on for Ronnow, will likely take his place.
Centre-back Doekhi and Japanese international Genki Haraguchi will likely come in after being rested at the weekend. Doekhi's impressive form this season has been vital for Union and his return in particular will be very much welcomed by Die Schlosserjung.
Having started two games in a week during this hectic schedule, Fischer may look to rotate wing-backs Christopher Trimmel and Julian Ryerson and midfielder Janik Haberer.
Their replacements would come in the form of the experienced Niko Giesselmann, Polish international left-back Tymoteusz Puchacz and the Hungarian Andras Schafer.
Meanwhile, Augsburg will welcome back their midfield pivot of Carlos Gruezo and Elvis Rexhbecaj. The duo missed the weekend with multiple-booking bans and their return will no doubt be a relief for Augsburg fans.
Furthermore, Polish right-back Robert Gumny may be in contention to start against Union, a move that will free up Jensen's attacking output down the right flank for Augsburg.
Having come on for left centre-back Mads Pedersen this weekend, Felix Uduokhai may be used to combat Union's aggression.
Union Berlin possible starting lineup:
Grill; Doekhi, Knoche, Leite; Giesselmann, Haraguchi, Khedira, Schafer, Puchacz; Michel, Becker
Augsburg possible starting lineup:
Gikiewicz; Gumny, Bauer, Gouweleeuw, Uduokhai; Jensen, Gruezo, Rexhbecaj, Niederlechner; Berisha, Demirovic
We say: Union Berlin 2-2 Augsburg
Though there is a large divide between these two teams in terms of results this year, the potential for a goal-fest is very high between Union and Augsburg.
The fluid counter-attacking of Augsburg's formation, including an in-form Mergim Berisha, the rapidity of Jensen on the wings, and the intelligence of Demirovic and Niederlechner, could all help to exploit Union's aggression.