The Gunners announced his injury on Monday along with their expectation that he will return to full training in early February.
Mkhitaryan last featured for Arsenal in their 2-0 defeat to Tottenham Hotspur in the Carabao Cup on December 19, in which he was withdrawn at half-time.
Football.London's Charles Watts believes he has pinpointed the moment Mkhitaryan suffered the injury:
In his final campaign with the Bundesliga side, he contributed a phenomenal 23 goals and 32 assists as he showcased his ability to operate at a high level on either flank or as a No. 10.
The Armenia international hasn't been close to that player for the Gunners, so his absence is perhaps not a devastating loss to their ambitions.
Nevertheless, Mkhitaryan—who scored twice against Southampton in the match prior to facing Spurs—has the potential to be a match-winner for Arsenal, and the team are denied that option while he's out.
If he's ever to recapture his best form, enforced spells on the sidelines will do him little good.