The Gunners released an update on their official website confirming the news.
The 25-year-old left-back sustained the knee injury in their pre-season clash against Chelsea earlier this week and had to be helped off the pitch.
The North London club have also confirmed that attacking midfielder Aaron Ramsey developed a tightness in his calf after the same fixture but the club hopes that he can return to training next week.
This is a shattering blow for new Arsenal boss Unai Emery, who is having to face the prospect of being without two senior players for their opening fixture against Manchester City. Facing the Premier League champions was going to be hard enough as it is without the injuries to contend with. Luckily, Nacho Monreal can play at left-back to cover the injured 25-year-old and Ramsey can be covered by the new arrival from Sampdoria Lucas Torreira in midfield but it’s still not ideal preparation. There is a sense of optimism for the new season at The Emirates, with a new manager at the helm and a raft of new experienced players, mixed with their promising young talents, there is some genuine excitement building about what Arsenal can do this year. Thing’s got very bad towards the final weeks of former manager Arsene Wenger’s tenure, surely the only way from there is up.