Real Madrid could only manage a 1-1 draw at the Santiago Bernabeu as Barcelona replaced them at the top of the table with a superior goal difference.
Vinicius Junior gave Los Blancos the lead on the stroke of halftime but Kike Garcia responded for the visitors, who had David Garcia sent off with just over ten minutes remaining for a push on Benzema, resulting in a penalty.
The Frenchman, who was returning from a three-week injury layoff, crushed his effort against the crossbar and Madrid could not find a winner thereafter.
The 34-year-old had not featured for Real Madrid since hobbling off in their opening Champions League group stage game against Celtic on September 6, missing matches against Real Mallorca, RB Leipzig and Atletico Madrid.
Benzema also missed France's Nations League games against Austria and Denmark as a result.
Ancelotti refused to blame Benzema for the European champions' first dropped points of the campaign and was satisfied with the output of his team in general.
"Karim usually scores," Ancelotti told DAZN.
"The match wasn't spectacular, but after the goal we conceded, the team played as it had to play.
"Karim had no problems starting to play 90 minutes. He has to get in the best shape by playing.
"It's a footballing accident that can sometimes happen. We've been quite solid at the back, we had a good balance, we hardly conceded any counter-attacks."
Benzema has scored three goals in five La Liga games this season.
