With scores deadlocked at 1-1 after Joselu’s 43rd minute goal cancelled out Vinicius Junior’s 12th minute opener, Karim Benzema struck twice late.
Rodrygo, who entered the game in the 58th minute for Federico Valverde, lofted a back-post cross for Benzema to score with a cushioned volley in the 88th minute.
Benzema added another with a classy free-kick after chaos on the edge of the box which resulted in a red card to Espanyol goalkeeper Benjamin Lacomte.
Camavinga came on for Luka Modric alongside Rodrygo in a double change, which Ancelotti felt was decisive.
“When he enters a broken game, his energy helps a lot,” Ancelotti told reporters.
“We won the game because of that energy. That came from Camavinga, Rodrygo and Ceballos. The game was slow and we changed it.”
Ancelotti added: “We had a very good start. We controlled the game well and struck early.
“We then dropped our intensity a bit, we weren’t effective up front and it became more difficult after they equalised because there were a lot of stoppages.
“The game became end to end and that suited us.”
Ancelotti’s decision not to use Eden Hazard raised some eyebrows, with the aforementioned trio preferred ahead of him.
“Hazard might have been an option, but I didn’t want to unbalance the team,” he said.
“We already had Rodrygo, Vinicius and Karim playing. Bringing on another forward could have been risky.
“He has my full backing and it’s not a problem if he doesn’t play, doesn’t get on the pitch or doesn’t warm up. He will get playing time.
“I’m not Hazard’s, Benzema’s, Mendy’s, Camavinga’s or Valverde’s coach. I’m Real Madrid’s coach.”