Should Bayern Munich beat Hertha Berlin on Saturday as expected, the visitors will require all three points in order to move back to the top of the table.
Match preview
Xabi Alonso's search for a second win as Bayer Leverkusen boss continues after drawing 0-0 at home to Club Brugge in the Champions League on Tuesday, although avoiding defeat against the Belgians did at least ensure that they will compete in the Europa League in the New Year after finishing above Atletico Madrid on goal difference in Group B.
In the meantime, though, Die Werkself's main focus must be on improving their league position, with last weekend's 2-0 defeat away to RB Leipzig leaving them third from bottom in the table and two points adrift of safety.
With Hertha one of the two sides sitting two points above in the table, there is a strong chance that Alonso's side can move outside of the bottom three by beating Union Berlin on Sunday given that Hertha host in-form Bayern at the Olympiastadion 24 hours prior.
However, on current form beating Union Berlin looks a daunting task given that they have won eight of their 12 league games so far this season, but there is little doubt that Leverkusen possess the attacking firepower to surpass the majority of teams in the Bundesliga on their day.
Indeed, very few people would have predicted both teams being in the positions that they currently are more than one-third into the league campaign, with Danilho Doekhi's injury-time winner against Borussia Monchengladbach last weekend keeping Urs Fischer's side at the summit.
Die Eisernen looked set to fall to their first home defeat in the Bundesliga since February with Nico Elvedi's header keeping the visitors ahead until Kevin Behrens responded in the 79th minute with a powerful header himself, before Doekhi sent Stadion An der Alten Forsterei into delirium with almost the last kick of the game.
Fischer's side's season continued to go from strength-to-strength as they secured their place in the Europa League knockout stages courtesy of a 1-0 win away to Belgian side Union SG on Thursday, with Sven Michel justifying a recall to the starting XI by scoring the crucial goal in the sixth minute.
As is often the case, Union Berlin never looked likely to give up their lead such is their defensive resilience and organisation under Fischer's management, and they will be hoping to pile on the misery for Leverkusen on Sunday by starting quickly once again.
Bayer Leverkusen Bundesliga form:
D
L
W
L
D
L
Bayer Leverkusen form (all competitions):
L
L
D
D
L
D
Union Berlin Bundesliga form:
W
L
W
W
L
W
Union Berlin form (all competitions):
W
W
L
W
W
W
Team News
Leverkusen will remain without long-term absentees Florian Wirtz, Sardar Azmoun and Charles Aranguiz due to injury, with the trio unlikely to return prior to the World Cup starting later this month.
Karim Bellarabi is also a doubt due to illness, but Piero Hincapie is available once again having missed out due to suspension in midweek and could be recalled to the starting XI alongside Kerem Demirbay.
Union Berlin, meanwhile, will travel without Kevin Mohwald due to a lack of match fitness, while Andras Schafer is unlikely to start after suffering a knock against Union SG on Thursday night.
Jordan Siebatcheu received a rest and did not travel to Belgium, but he should return to the side despite Michel laying a claim for his place by scoring the decisive goal which sent them through in the Europa League.
Bayer Leverkusen possible starting lineup:
Hradecky; Tah, Tapsoba, Hincapie; Frimpong, Andrich, Demirbay, Bakker; Diaby, Schick, Hudson-Odoi
Union Berlin possible starting lineup:
Ronnow; Doekhi, Knoche, Leite; Trimmel, Haraguchi, Khedira, Haberer, Ryerson; Becker, Siebatcheu
We say: Bayer Leverkusen 1-1 Union Berlin
Leverkusen could benefit from having an additional two days' rest compared to their opponents, who are surely set to lose hold of top spot sooner or later given the form of Bayern since the return from September's international break.
The goals have dried up in recent weeks for Siebatcheu and Sheraldo Becker, meaning Union Berlin may have to settle for a share of the spoils on Sunday.