Two late goals by 19-year-old Nico Paz and Joselu gave Real a 4-2 home win against Napoli on November 29th as the 14-time European champions maintained their 100% record with 15 points from five games, eight points ahead of the Italian champions.
"The goal is achieved, we have done well so far. But we want to finish well, showing a good image," Ancelotti told a press conference on Monday.
"As we always say, it's a special competition, we have to respect it. Our professionalism is (also) at stake. We have to play a good game."
Union will take on Real under new coach Nenad Bjelica, who was appointed last month to replace the sacked Urs Fischer following their nine-game losing run in the Bundesliga.
They are bottom with two points, two points behind Braga, but can finish third and enter the Europa League knockout round playoffs if they beat Real and Napoli defeat the Portuguese.
"(Bjelica) has changed the system, the game strategy, he is going to play more offensive than at the Bernabeu," Ancelotti said. "Every coach has his ideas. In football there are no secrets, it will be an entertaining and even match."
Real goalkeeper Kepa Arrizabalaga will return to Real's starting lineup with Andriy Lunin having taken over as Ancelotti's first choice - in LaLiga and the Champions League - from their 3-0 home win against Braga in early November.
"Kepa will play, fair or unfair," Ancelotti said when asked about the competition between the two keepers.
"I don't know if it's unfair to leave someone on the bench, it would be unfair every game. Someone has to stay on. The good thing is that Lunin has had the opportunity that he didn't have in the past to show his characteristics, we are very happy.
"He did well. Kepa didn't have to show it, he has a track record. Lunin's last period has been good, but you have to bear in mind that Kepa has always done well."
The Italian also praised 24-year-old attacking midfielder Brahim Diaz, who was on loan at AC Milan in 2020-2023 and has scored twice in LaLiga this season.
"The time spent in Milan did him a lot of good, defensively he has worked a lot and he has returned in an optimal tactical condition," said Ancelotti.
"He always positions himself well without the ball. It's rare to see this in a striker of his quality. It's usually more difficult for players like him to position themselves without the ball."
