The Chilean national team revealed the nature of Sanchez's problem via their official website on Tuesday (h/t the Daily Express' Uche Amako). According to the Sun's Daniel Cutts the Gunners have been informed of the likely timeframe for his recovery, though he has still yet to undergo a scan of the injury.
Sanchez, 27, looks set to miss crucial upcoming Gunners fixtures against Manchester United and Paris Saint-Germain as well as much of the busy Christmas period, perhaps including Manchester City on December 18.
Sanchez is one of the Gunners' most important players and is crucial to the effectiveness of the Arsenal forward line.
He has netted six goals and provided three assists in the Premier League so far this season, per WhoScored.com.
His linkup play with Mesut Ozil can be devastating at times, and he has the ability to turn a game on its head with a single swing of his right boot.
Arsenal have adequate depth to cover for him in attack, but the former Barcelona man is the player who makes the Gunners tick.
The likes of Ozil, Alex Oxlade-Chamberlain, Theo Walcott and Olivier Giroud will need to step up to create the chances and score the goals that will be lost in Sanchez's absence.
Manager Arsene Wenger will be eager to see him return to action as soon as possible, as he will be vital with Arsenal battling for the Premier League's top spot.