The visitors finally ended their 12-match losing streak in all competitions by drawing away to Napoli in the Champions League on Wednesday, but they will head into this match occupying the relegation places.
Match preview
Leverkusen, on the other hand, secured their 11th consecutive victory in all competitions by winning 1-0 against Qarabag FK in Azerbaijan on Thursday, which booked their place in the last-16 of the Europa League as winners of Group H.
Indeed, Xabi Alonso's side will bypass the last-32 stage of Europe's secondary competition - which takes place in February - meaning they can fully focus on maintaining their status at the top of the Bundesliga, as well as their progression in the DFB-Pokal over the next few months.
Die Werkself were pegged back away to Hoffenheim last weekend, with quickfire strikes from Anton Stach and Wout Weghorst before the hour mark cancelling out Florian Wirtz and Alex Grimaldo's efforts in the first half. However, Grimaldo was on hand once more with 20 minutes of normal time remaining to secure a crucial victory for his side.
Much like his compatriot in charge, the Spaniard has been a revelation since joining from Benfica during the summer transfer window, scoring five goals in his last seven league games from left wing-back. With Jeremie Frimpong always presenting an attacking threat from the opposite flank, too, it is clear to see why this Leverkusen side are scoring so freely.
Alonso's only concern may be that his team have began to concede more goals in recent weeks - they have only kept one clean sheet in their last six games - but if they can continue to attack with such potency, this should not stop them in their tracks any time soon.
Having been defeated in 12 successive matches prior to their trip to Naples in midweek, Union Berlin could hardly be in more contrasting form to their forthcoming opponents on Sunday. However, Urs Fischer's side showed serious grit and defensive resilience to claim a point at Napoli on Wednesday, finally putting an end to their losing streak.
Matteo Politano had provided the hosts with the lead six minutes before half-time, with Die Eisernen staring down the barrel at another defeat. However, David Datro Fofana equalised seven minutes after the break, with Napoli unable to find a breakthrough despite generating 27 shots across the course of the encounter.
While only drawing means Union Berlin can no longer progress to the knockout stages in their maiden Champions League appearance, it does keep their hopes of remaining in European competition alive. If they can defeat Braga in Portugal on the next matchday, they will leapfrog their opponents into third place in Group C, meaning dropping down to the Europa League will be in their own hands.
However, Fischer will be more concerned with his side's unexpectedly low league position having made astonishing progress in recent years, with an eighth successive league loss away to Eintracht Frankfurt last weekend leaving them in the bottom three. With Mainz 05 and FC Koln - the two sides beneath them prior to any action taking place this weekend - facing potential relegation rivals SV Darmstadt 98 and VfL Bochum respectively on Saturday, there is a chance that Union Berlin could travel to Leverkusen at the foot of the table.
While Fischer will not be placing too much pressure on his players given their opponents' excellent form, Union Berlin's display in midweek proved that they can still defend to an elite level, and they are going to need all of those battling qualities to dig themselves out of this very deep hole which they now find themselves in.
Bayer Leverkusen Bundesliga form:
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Bayer Leverkusen form (all competitions):
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Union Berlin Bundesliga form:
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Union Berlin form (all competitions):
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Team News
Leverkusen will remain without Arthur and Patrik Schick due to muscular injuries, while Edmond Tapsoba is a doubt due to a hand injury. Piero Hincapie would be the obvious candidate to replace Tapsoba on the left side of the hosts' back three, but Odilon Kossounou could switch sides with Josip Stanisic earning a recall.
Having scored two or more goals in all 10 league games so far this season, it would be a surprise if Alonso made any changes to his front seven positions from the victory against Hoffenheim last weekend. However, after scoring the winning penalty against Qarabag on Thursday, Victor Boniface will be keen to get back on the scoresheet in the Bundesliga having blanked in his last three league starts.
Union Berlin, meanwhile, made five changes for their 1-1 draw against Napoli in midweek, with the fact that Fischer rested the likes of Robin Gosens and Robin Knoche suggesting that his priority is understandably rescuing his side's league position.
Those two are likely to come back into the visitors' starting XI, but having produced an improved performance, the 57-year-old may keep faith with the rest of his players. As such, Fofana and Sheraldo Becker may be preferred to Kevin Behrens in attack.
Bayer Leverkusen possible starting lineup:
Hradecky; Kossounou, Tah, Hincapie; Frimpong, Palacios, Xhaka, Grimaldo; Hofmann, Wirtz; Boniface
Union Berlin possible starting lineup:
Ronnow; Bonucci, Knoche, Leite; Juranovic, Laidouni, Khedira, Haberer, Gosens; Becker, Fofana
We say: Bayer Leverkusen 2-1 Union Berlin
In terms of form, this could be the biggest mismatch in European football this weekend, but that does not paint the full picture. Union Berlin are a much better team than they have shown so far this season, and their confidence will have been boosted by drawing away to Serie A champions Napoli in midweek.
However, Leverkusen are a serious attacking force at present and have more strings to their bow in terms of breaking their opponents down. While we can envisage the visitors making Leverkusen work hard for their victory, especially having travelled back from Azerbaijan less than 72 hours prior to this fixture, they should have enough to get over the line in the end and remain at the top of the table.