Monreal had a baptism of fire to English football against Stoke City on his debut, but Sagna says the 26-year-old's performance proved he can be a long-term success.
"I didn't know much about him [before he came to Arsenal]," Sagna told Arsenal Player. "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.
"It was a difficult game to play in, with a lot of physicality and a lot of challenges. We won and everyone is happy."