Los Blancos will enter the contest off the back of a 2-1 victory over Real Sociedad in La Liga, while Union Berlin suffered a 2-1 loss to Wolfsburg in their last Bundesliga fixture on Saturday.
Match preview
Real Madrid have not quite been at their free-flowing best early in the season, but the capital giants have been incredibly effective, recording five straight wins in La Liga to sit top of the table on 15 points.
Carlo Ancelotti's side beat Athletic Bilbao and Almeria by two-goal margins in their opening two league games of the campaign, but they have only edged their last three matches against Celta Vigo, Getafe and Real Sociedad, beating the latter 2-1 on Sunday evening to return to the summit.
Real Madrid have the small matter of a trip to Atletico Madrid in the Madrid derby this weekend, but their current focus is on the Champions League, as they bid to make the perfect start to a section which also includes Portuguese outfit Braga and Italian champions Napoli.
The Whites have won the European Cup an incredible 14 times, making them comfortably the dominant force in the history of this competition, with their last success coming in the 2021-22 season.
Real Madrid reached the semi-finals of last season's competition, losing to Manchester City, and they will again be expected to advance deep into the tournament this term.
Union Berlin, meanwhile, finished fourth in the Bundesliga last season, claiming 62 points from their 34 matches, which saw them directly qualify for the group stage of the Champions League for the first time.
Die Eisernen played in the Europa Conference League during the 2021-22 season but failed to progress past the group stage, but they did reach the last-16 stage of the 2022-23 Europa League.
Union Berlin will now make the step up into the biggest cup competition in European football, and they will enter the contest at Bernabeu of the back of two disappointing domestic results.
Indeed, Urs Fischer's side have lost their last two in the Bundesliga against RB Leipzig and Wolfsburg, which followed three straight wins in all competitions, including their opening two in the league, and a total of six points has left them in eighth position in the table.
Union Berlin are very much an unknown quantity in this competition, which could help them on Wednesday evening, but their home form in the section is likely to determine whether they have a genuine chance of reaching the knockout round of this season's tournament.
Real Madrid form (all competitions):
Union Berlin form (all competitions):
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Team News
Real Madrid managed to recover one injured player at the weekend, with Dani Ceballos in a matchday squad for the first time this season, having overcome a hamstring issue.
Ferland Mendy could be back for this contest, but Los Blancos will again definitely be without the services of key trio Thibaut Courtois, Eder Militao and Vinicius Junior, while summer arrival Arda Guler is also still unavailable.
Joselu has impressed alongside Rodrygo in the final third of the field, and the pair are set to continue their partnership on Wednesday, while it is likely to be the same back five from Sunday.
Ancelotti could potentially make just the one change, with Eduardo Camavinga likely to be recalled in place of Toni Kroos, who is no longer a guaranteed starter, which is also the case for Luka Modric.
Jude Bellingham will get his first taste of Champions League football in a Real Madrid shirt, meanwhile, and the 20-year-old has been in excellent form this season, already finding the back of the net on five occasions.
Union Berlin, meanwhile, will be without the services of Rani Khedira, Andras Schafer and Laurenz Dehl for the contest in the Spanish capital through injury.
Kevin Volland is currently serving a Bundesliga suspension, but the forward is available for this contest and will more than likely provide an option off the bench.
Kevin Behrens has been the team's main source of goals early in the season, finding the back of the net on four occasions, and he will again feature in the final third of the field.
There is also likely to be a start for on-loan Chelsea forward David Datro Fofana, although the experienced Leonardo Bonucci is set to feature among the substitutes for the first whistle.
Real Madrid possible starting lineup:
Kepa; Carvajal, Rudiger, Alaba, F Garcia; Valverde, Tchouameni, Camavinga; Bellingham; Joselu, Rodrygo
Union Berlin possible starting lineup:
Ronnow; Doekhi, Knoche, Leite; Juranovic, Laidouni, Kral, Aaronson, Gosens; Fofana, Behrens
We say: Real Madrid 3-1 Union Berlin
This has the potential to be a very entertaining match, as Union Berlin will look to make their mark on Wednesday evening. We are backing the visitors to score but are fully expecting Real Madrid's vast experience in the competition to help them navigate their way to all three points in the Spanish capital.