The visitors, meanwhile, moved five points clear of the relegation zone by dispatching SV Darmstadt 98 last weekend.
Match preview
The winter break appeared to have arrived at the worst possible moment for Leipzig, who had enjoyed their Christmas dinners and New Year celebrations with a six-point lead ahead of Borussia Dortmund inside the Champions League qualification positions. Yet three consecutive losses means they head into Sunday's fixture outside of the top four positions in the table.
Their latest defeat was a particularly bitter pill to swallow, as they were thrashed 5-2 by third-placed Stuttgart. Benjamin Sesko had pulled one back for the visitors to trail 2-1 at half-time, but Deniz Undav's hat-trick blew Die Roten Bullen away in the second half.
With Dortmund travelling to Heidenheim on Friday evening, Leipzig are likely to find themselves at least four points off the Champions League qualification spots, meaning they must return to winning ways sooner rather than later if they are to qualify for Europe's elite competition for the sixth successive season.
Of course, Marco Rose's side have an unenviably difficult task when facing Real Madrid in the last-16 later this month, which on current form could see them blown away. Yet if they can produce positive results and performances against Union Berlin and Augsburg between now and then, their outlook could be much different.
Union Berlin, meanwhile, were eliminated at the group stage in their first ever outing in the Champions League earlier this season. After famously finishing fourth under Urs Fischer, their legendary manager has since been sacked following a disastrous start to the campaign.
Nenad Bjelica appears to have steadied the ship somewhat since being appointed in late November, winning three and drawing one of his six league games in charge. Given that one of the other games was a narrow 1-0 defeat to Bayern Munich, the only major blemish on Bjelica's copybook so far was a 3-0 defeat away to VfL Bochum.
Benedict Hollerbach's strike ensured that Die Eisernen edged out Darmstadt in a crucial relegation six-pointer last Sunday, with Union Berlin amassing 21 shots across the course of the encounter. However, their rock-bottom opponents had plenty of chances of their own, with the hosts surviving a nervy finish.
Having failed to score in each of their last five away trips in the Bundesliga - only managing to avoid defeat in a stalemate against Freiburg last month - Union Berlin are clearly up against it to claim another positive result against Leipzig. However, they are arguably facing them at the best possible moment, even if they will have to watch the three teams beneath them in the table all play on Saturday first of all.
RB Leipzig Bundesliga form:
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RB Leipzig 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
Leipzig will be without Mohamed Simakan, who must serve a one-match suspension after receiving his fifth yellow card of the season against Stuttgart. Benjamin Henrichs is likely to come into the hosts' side at right-back.
Xavi Simons should return from a recent injury problem to start from the right flank, but Amadou Haidara remains away with Mali at the Africa Cup of Nations.
Union Berlin, meanwhile, will travel without Josip Juranovic due to a muscle strain. Kevin Behrens, Jerome Roussillon, and Kevin Volland are all doubtful due to their own issues, potentially leaving Bjelica short of attacking options.
Behrens was replaced by Mikkel Kaufmann in injury time against Darmstadt due to a knee problem, but Chris Bedia may receive the nod if the Germany forward misses out as expected.
RB Leipzig possible starting lineup:
Blaswich; Henrichs, Klostermann, Lukeba, Raum; Simons, Seiwald, Schlager, Olmo; Sesko, Openda
Union Berlin possible starting lineup:
Ronnow; Knoche, Vogt, Leite; Trimmel, Tousart, Kral, Schafer, Gosens; Bedia, Hollerbach
We say: RB Leipzig 2-1 Union Berlin
Union Berlin have rediscovered their defensive solidity since appointing Bjelica, but their away form remains rotten this season. Leipzig, meanwhile, have struggled since returning from the winter break, but are surely too good for their disastrous form to continue much longer.
As such, we expect Rose's side to return to winning ways on Sunday, but they are likely to be made to work extremely hard for their victory.