The Gunners have kept clean sheets in their last two Premier League games that Monreal has featured in and Sagna believes that some of the credit should go to the club's new Spanish full-back.
"I didn't know much about him [before he came to Arsenal]," Sagna told the club's official website. "I had a chance to speak with Santi Cazorla and he talked about him. He looks good. He had a great game in his first game.
"It's not easy because he only trained with us once before. I wouldn't like to be in his position, but he did well."
Arsenal are believed to have parted with £8.5m to sign the 26-year-old.