The Gunners have enjoyed a solid start to their Premier League campaign, with one win and two draws moving them up to seventh place in the table.
However, with players coming back late from the World Cup and eight new signings to bed in, Monreal believes that Arsenal's best form is still to come.
"I think we need a little bit more time because last summer a lot of players were playing in the World Cup and they came back late," he told the club's official website. "We have signed four or five new players so maybe we need a little bit more time for the players to feel better and their fitness can feel better. I think in the next weeks the team will play better."
Arsenal continue the new Premier League season on Saturday when they welcome champions Manchester City to the Emirates Stadium.