The Los Blancos manager saw his substitutes come good in the closing stages of the game to pave the way for Benzema's heroics.
Regarded as a humble man, this time Ancelotti was happy to take credit for the decisions that saw him shuffle his Madrid pack in the second half of a hard-fought contest.
He brought off Luka Modric and Federico Valverde in the 58th minute, introducing Eduardo Camavinga and Rodrygo, before replacing Toni Kroos with Dani Ceballos.
Camavinga brought fresh vibrancy to the Madrid midfield, Rodrygo set up Benzema's first goal in the 88th minute, and Ceballos was fouled for the free-kick that led to a red card and Benzema's late clincher.
That final goal of the game came 10 minutes into stoppage time, after a marathon delay for the VAR to resolve the moment when home goalkeeper Benjamin Lecomte clattered Ceballos. Lecomte was sent off and home captain Leandro Cabrera, a defender, took over in goal, Benzema taking advantage by firing home from the set-piece.
"We won the game because of that energy. That of Camavinga, Rodrygo or Ceballos," Ancelotti said. "The game was slow and we changed it. Each game has its own story."
He said it was impossible to know how the game would have flowed had he fielded his substitutes from the start, allowing the likes of Modric and Kroos to come on and change the game in the second half.
"Sometimes team selection is a gamble and today I think the Real Madrid coach has gambled well."
The veteran Italian coach said Madrid had started brightly, as was exemplified by the sharp movement that brought about Vinicius Junior's early goal.
Former Madrid reserve striker Joselu equalised with his seventh goal in nine LaLiga matches against Los Blancos, before Benzema's double provided the late twist.
Three days after picking up the UEFA men's player of the year award, Benzema showed why he is earning such accolades at this stage of his career. He turns 35 in December but shows no sign of slowing down.
Benzema has scored 28 goals for Madrid in 2022 across all competitions, the joint highest of all players from Europe's top five leagues. It puts him alongside Paris Saint-Germain's Kylian Mbappe, who snubbed a move to Madrid in favour of signing a new PSG deal in May.
In all, Benzema had eight shots, the highest of any player in a single game during the opening weeks of the LaLiga season.
The late show poured salt on Espanyol wounds, with the team from Barcelona having now failed to win in its last 10 league games, going back to last season.
Kroos told Movistar: "It's to be expected that we suffer against a great rival, also away from home. We controlled the game until their goal. We suffered a bit in the second half, but in the last 30 minutes we regained control.
"They looked tired and we took advantage of that. Winning away is not easy and we have done it three times in a row."
Madrid's first three league assignments have indeed come away from the Santiago Bernabeu, while work is carried out on the stadium, and wins over Almeria, Celta Vigo and now Espanyol have made it an ideal start.
Madrid sits top of the table but is joined on nine points by Real Betis, the team it hosts next Sunday (AEST).
Kroos says Madrid is braced for a busy run of games, as the Champions League group stage begins.
"Every three to four days we are going to play against difficult opponents and there are always times when we are going to have to suffer," Kroos said.