Benitez was relieved of his duties at Madrid after just seven months into a three-year deal in the Spanish capital.
The Spaniard had helped the club top their Champions League group and was just four points behind La Liga leaders Atletico Madrid when he was relieved of his duties, but Ancelotti, who was sacked by Los Blancos a year after guiding the club to their 10th European Cup trophy, hit out at the decision to let Benitez go.
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"After many games this season, Real Madrid are still fighting for the titles in La Liga and the Champions League, but the president Florentino Perez decided to say goodbye to Benitez," Ancelotti told Sina Sports.
"I know the job of a coach is always about getting results.
"I also know our work is subject to debate, but this time I really cannot understand the decision. It's only half-way through the season."
Zinedine Zidane was appointed Benitez's successor at Madrid, and the Frenchman's first game in charge saw the club ease past Deportivo La Coruna 5-0 on Saturday.
"Zidane is Madrid's fifth coach since 2009, and there are questions that need to be raised," Ancelotti continued.
"Is it always the coach's fault? Everyone can draw their own conclusions.
"But without doubt, Zidane has the sufficient capacity to lead Real Madrid. I sincerely wish him the best."