Los Blancos booked their spot in the last-16 stage of the competition courtesy of a 1-0 victory over their fourth-tier opponents, with Rodrygo netting the only goal of the contest at Estadio Principe Felipe.
However, it was a struggle for the Spanish and European champions, who found it difficult to get to grips with the surface in Extremadura.
Ancelotti has said that Cacereno's stadium "needs more work" and praised his players for navigating their way through a difficult last-32 contest.
"We couldn't play the way we planned because of the state of the field. We are not used to playing like this but I am satisfied with today's performance. The result is deserved," the Italian told reporters.
"It's difficult to fully assess the game because it was difficult to play tonight due to the pitch. [Eden] Hazard and Rodrygo faced more difficulties and could not create many chances, but fortunately, Rodrygo managed to finish the match with a goal.
"You can't play football like that. This is not football and the stadium needs more work. Such things should be monitored by the officials. It's good to see the kids fight and compete against the big boys. That's good for the fans, but they want to see good football.
"I liked the technical side of the players, I knew we had to struggle in this environment. This is our second clean sheet and that makes me very happy, we didn't suffer as we expected and everything was fine."
The Copa del Rey is third on the club's list of priorities this season behind La Liga and the Champions League.
However, Ancelotti has said that his side are ready to "compete in all competitions" this term.
"We are a team that can win and compete in all competitions. Our genes are not to give up and we will compete for all championships as best as possible," said the Italian.
Real Madrid have won the Copa del Rey on 19 occasions, making them the third-most successful team in the history of the competition behind Athletic Bilbao (23) and Barcelona (31).
However, Los Blancos have only won the tournament three times since 2004.
Ancelotti's side will be bidding to make it four straight wins in all competitions when they travel to Villarreal in Spain's top flight on January 7 before taking on Valencia in the semi-finals of the Spanish Super Cup three days later.