The France international broke a bone in his foot during the 2-2 draw against Everton on Saturday, which could force Arsene Wenger to buy a new striker.
A specialist confirmed the France international will be out at least until November after he assessed detailed scans of the Frenchman's ankle on Tuesday.
It is a huge blow to Arsene Wenger, who is sweating over the fitness of Alexis Sanchez for the make-or-break Champions League qualifier against Besiktas.
Sanchez is carrying a knock sustained in the 2-2 draw at Everton on Saturday, although he is expected to pull through for the second-leg clash at the Emirates Stadium.
However, Giroud is preparing for a lengthy absence, which could force Wenger to make a late addition to his strike force in the final days of the transfer window.
The initial scan failed to clear up the extent of the damage to an ankle injury suffered by Giroud during the closing stages of Arsenal's draw at Goodison Park.
He was sent for a second examination after the inflammation of the joint had subsided, which revealed a broken bone in his foot.
A specialist has confirmed the worst fears of Arsenal's medical team and Giroud is now set for a lengthy spell of rehabilitation.
He is regarded by the club as a quick healer but is expected to be out for up to 10 weeks.
Wenger now has only Sanchez, who led the forward line in the first half agajnst against Everton before going off injured, and unproven pair Yaya Sanogo and Joel Campbell in attack for the early months of the season.
He is likely to dip into the market for a new forward but his plans will depend upon whether Arsenal can overcome Besiktas on Wednesday night and progress into the lucrative Champions League group stages.
Arsenal's plans to recruit a new center back already suffered a blow on Tuesday after the club lost out to Roma in the race to sign Greece international Kostas Manolas.
He was undergoing a medical at the Serie A club on Tuesday after it trumped the Gunners by offering 13 million euros for the defender.