Los Blancos have already topped Group C, while Union Berlin are bottom of the section, and the hosts must beat Carlo Ancelotti's side and hope that Braga lose to Napoli in order to claim third, which would see them secure a spot in the Europa League knockout round playoffs.
Nenad Bjelica has made a successful start to life as head coach of Union Berlin, with the 52-year-old leading the team to one win and one draw since arriving at the end of November.
Saturday's 3-1 success over Borussia Monchengladbach in the Bundesliga proved to be the team's first victory since the end of August, with the result moving them into 15th position in the table on 10 points.
Die Eisernen picked up a point in a 1-1 draw away to Braga in the Champions League last time out, which kept their hopes of finishing third in the section alive. Indeed, if Union Berlin beat Real Madrid, and Braga lose against Napoli, then the German team would finish third and secure a spot in the Europa League.
Union Berlin finished fourth in the Bundesliga last season, which saw them directly qualify for the group stage of the Champions League for the first time, but they have found it difficult coming up to the top table, picking up just two points from five matches, which has left them at the foot of the section.
The Iron Ones have won three of their last five home European matches, and they will be welcoming a Real Madrid side that have lost 20 of their 35 previous away matches against German opposition, which should hand the hosts confidence ahead of what is a very tricky match, at least on paper.
Real Madrid made it five wins from five Champions League matches this season when they recorded a 4-2 victory over Napoli at Bernabeu on November 29, with the result seeing them advance to the knockout round of the competition as section winners.
Los Blancos have been impressive in this season's competition, but they only just edged past Union Berlin in the reverse match at Bernabeu on September 20, with Jude Bellingham netting the winner in the 90th minute.
Real Madrid will enter this match off the back of a 1-1 draw with Real Betis in La Liga, with the result leaving them at the top of the table on 39 points, and they have two more league games against Villarreal and Alaves to come before the winter break in Spain's top flight.
The capital giants are bidding to win all six of their matches in a Champions League group for a record-equalling third time, and they are comfortably the most successful side in the history of this competition, lifting the trophy a staggering 14 times, with their last win coming in 2021-22.
Real Madrid have lost eight of their last 25 Champions League away matches, though, including four defeats in their last 11, and the fact that they are already through to the next round could make this a very interesting match, as Union Berlin need to go for the win.
The Bundesliga side are otherwise in strong shape, and there are not expected to be any real surprises in their XI, with Kevin Behrens, who has four goals to his name this season, set to lead the line.
Head coach Ancelotti may decide to rest Bellingham from the start in this match, as the midfielder is nursing both a shoulder and an ankle problem, so this could be the perfect chance to give him a night off.
Nacho, Kepa Arrizabalaga and Dani Ceballos could also come in, while there might be a spot in the final third for Joselu, with Ancelotti set to rotate following the clash with Real Betis at the weekend.
Union Berlin possible starting lineup:
Ronnow; Juranovic, Knoche, Leite, Roussillon; Volland, Tousart, Khedira, Laidouni, Gosens; Behrens
Real Madrid possible starting lineup:
Kepa; Vazquez, Rudiger, Nacho, Mendy; Valverde, Ceballos, Modric; Diaz; Rodrygo, Joselu
We say: Union Berlin 1-2 Real Madrid
Union Berlin's last two results would have given them a lot of confidence, and they will fancy their chances of picking up a positive result on Tuesday. Real Madrid will be far from at full strength, but we are still expecting Ancelotti's side to claim all three points to make it six wins from six in the section.
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