According to Bio Bio Chile (h/t Jack Rathborn of the Daily Mirror), the Gunners attacker picked up the problem in training ahead of the team's clash with Bolivia; Sanchez is said to be a big doubt for that encounter on Tuesday.
Rathborn added that according to El Mercurio, the Arsenal man had an ice pack on his left ankle as he left the session.
"The medical staff continue to assess the player and are confident he can push through the pain barrier to feature at the Estadio Hernando Siles in La Paz," he continued. "Chile will likely risk Sanchez if they can due to their current predicament for Russia 2018."
Sanchez's potential absence will come as a major blow for manager Arsene Wenger after a summer spent fighting to keep the player at the Emirates Stadium, such is his importance to the club's success in attack.
After scoring 24 Premier League goals last season—just five fewer than Golden Boot-winner Harry Kane—Sanchez will be seeking an even bigger haul over the course of the 2017-18 season.
However, Arsenal will be sweating on his fitness ahead of the clash with Bournemouth at the Emirates Stadium on Saturday. After a sluggish beginning to the season, the injury fear comes at the worst possible time for the team.
Wenger will be glad he recruited striker Alexandre Lacazette from Lyon over the summer, as he'll lend a major hand in bridging the goals gap during Sanchez's absence.
In the meantime, the Gunners chief may look to the likes of Theo Walcott, Danny Welbeck and Olivier Giroud to provide cover in the final third while their South American star begins his recovery.