Despite Benzema's return from a three-week injury lay-off, Madrid produced a disjointed performance on Sunday, squandering a first-half lead when Kike Garcia cancelled out Vinicius Junior's fortunate opener.
Benzema had the chance to fire Madrid to a seventh consecutive league victory when David Garcia was sent off for a push on the striker six yards out, but the France star crashed his spot-kick against the crossbar.
Osasuna's 10 men held on for a point in comfortable fashion thereafter, and Benzema has now missed three penalties against Jagoba Arrasate's team in LaLiga this year.
Ancelotti refused to blame Benzema for the result, however, and insisted Madrid had not performed badly, saying: "Karim usually scores.
"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."
Osasuna has now claimed three draws against Madrid since the start of the 2020-2021 campaign, more than any other team in LaLiga.
Madrid's failure to find a winning goal means they remain behind Barcelona on goal difference at the summit, and midfielder Dani Ceballos says they lacked dynamism in Monday's (AEDT) draw.
"We wanted the three points, we tried until the end, but we have lost two points," he said. "We improved in the last minutes, but we have not been able to get the victory.
"We were not good facing the goal, nor with the ball, we lacked a bit of dynamism. We have another game on Wednesday [against Shakhtar Donetsk in the Champions League] to face in a good way."