Leverkusen boss Xabi Alonso oversaw a 1-0 victory against Heidenheim on April 5, and his team are second with 62 points - six behind Bayern - while Union are 13th with 33 points and emerged as 1-0 victors against Wolfsburg on April 6.
Match preview
The home side produced just two shots on target against relegation-threatened Heidenheim and created no big chances, while facing two themselves, and they only claimed all three points thanks to Emiliano Buendia's 91st-minute winner.
Alonso will be alarmed by the nature of his team's performance, especially as they had been eliminated from the semi-final of the DFB-Pokal four days earlier at the hands of third division side Arminia Bielefeld when they lost 2-1.
While the champions kept a clean sheet against Heidenheim, it was only the first time in eight games that they had prevented their opponents from scoring having conceded 14 goals in that time.
With just six matchweeks remaining in the top flight, Leverkusen must hope that Bayern drop points in at least three fixtures considering the Bavarians' goal difference of 54 is significantly greater than their figure of 29.
Die Schwarzroten have lost four of their past seven in all competitions - winning on three occasions - but they have triumphed in five of their last six in the league.
Though the hosts have suffered two defeats and a draw in three of their four most recent outings at BayArena, they achieved victory in 14 of their prior 16 at home, drawing their other two matches.
Union Berlin were largely excellent from a defensive perspective against Wolfsburg, but they were fortunate that Wolves striker Kevin Behrens somehow failed to equalise in the final minute of regulation time despite being less than a yard out from goal.
The visitors had not kept a clean sheet in their previous six games, but their form in the final third has improved considering they have scored six times in their last four, whereas they only managed 11 goals in their prior 17.
Die Eisernen will be looking to exact revenge for their 2-1 defeat at the hands of Leverkusen in the Bundesliga on November 30, but they are winless against their hosts in eight clashes, losing four and drawing four.
Boss Steffen Baumgart will come into the match confident given his team are undefeated in four matches, winning three times.
Union have also emerged as winners in their past two fixtures away from home - as well as three of their last four - but they lost all seven of the matches that they played on the road between October 30 and January 26.
Bayer Leverkusen Bundesliga form:
W
W
L
W
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W
Bayer Leverkusen form (all competitions):
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L
W
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W
Union Berlin Bundesliga form:
L
L
W
D
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W
Team News
Leverkusen could be boosted by the return of talisman Florian Wirtz in the next few days, but the attacker is likely to be absent on Saturday alongside Martin Terrier and Victor Boniface, so perhaps Amine Adli and Aleix Garcia will support Patrik Schick in attack.
Midfielder Exequiel Palacios is not scheduled to return from a groin injury until late April, making Granit Xhaka and Robert Andrich the most probable candidates to appear in the middle of the pitch.
In defence, Nordi Mukiele, Jonathan Tah and Piero Hincapie may feature in a back three given Jeanuel Belocian and Mario Hermoso will not be in the squad.
As for Union, midfielder Aljoscha Kemlein will play no further part this season, so perhaps Baumgart could select regulars Janik Haberer and Rani Khedira in a double pivot.
Wing-back Robert Skov's calf injury may lead to starts for Christopher Trimmel and Josip Juranovic either side of a back three.
Attacker Woo-Yeong Jeong is set to miss out, but Andras Schafer, Benedict Hollerbach and Andrej Ilic are available.
Bayer Leverkusen possible starting lineup:
Hradecky; Mukiele, Tah, Hincapie; Frimpong, Xhaka, Andrich, Grimaldo; Adli, Garcia; Schick
Union Berlin possible starting lineup:
Ronnow; Doekhi, Querfeld, Leite; Trimmel, Haberer, Khedira, Juranovic; Schafer, Hollerbach; Ilic
We say: Bayer Leverkusen 2-1 Union Berlin
Leverkusen's defensive frailty makes them unlikely to keep a clean sheet, and they may have to recover from yet another deficit if they are to achieve victory.
Written by
Lewis Nolan