The French right-back went to hospital after he landed awkwardly during the 2-1 derby defeat to Spurs with his boss earlier suggesting that the first signs did not look good
Sagna jumped for the ball with Spurs defender Benoit Assou-Ekotto before landing awkwardly and immediately signalling that he wanted to be taken off. He was later taken to hospital, and a club statement has revealed that a fracture has been sustained.
"The Club can confirm that Bacary Sagna sustained a fracture to the fibula in his right leg during the match against Tottenham Hotspur this afternoon," the statement on the club's official website read.
"Further detailed investigations and assessments will take place on Monday, during which time Bacary will see ankle specialists and the Arsenal medical team regarding the further management of his injury.
"It is expected that Bacary will be out for approximately three months and a surgical procedure cannot be ruled out at this stage.
"Everyone at the Club wishes Bacary a speedy recovery and look forward to seeing him playing again as soon as possible."
Gunners boss Arsene Wenger had previously admitted he was "very worried" about the injury.
He told the club's official website: "We are very worried about his injury. He is in hospital at the moment and it is too early to say but the first signs do not look good at all.
"It is the fibula in the ankle that has to be checked, that is what I have been told. We will know tonight. I think he was pushed quite hard by Assou-Ekotto and I'm not sure what happened to him.
"He might have landed the wrong way, I don't know. I will have to see it again. Straight away he wanted to be substituted."
Team-mate Jack Wilshere, meanwhile, offered his support to the Frenchman and Tweeted: "Oh no bad news about @Sagnaofficial wish you a speedy recovery mate! I will be in the gym with you!"