The Spanish defender’s arrival at the Club has coincided with Arsène Wenger’s side keeping clean sheets in each of their last two games.
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.”
While Sagna recognises it can be tough to acclimatise to the Premier League, the French international believes it is crucial for defenders to settle quickly.
“As a defender, you have to run from the beginning to the end," he said. "When you’re a striker, you can give the maximum and maybe after 70 minutes, you will go out.
"It’s really rare for a defender to go out. You need to be at your best all of the time and you need to be competitive."Copyright 2017 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source