United shared the news via their official Twitter account:
The Chile international has endured a difficult stay at Old Trafford and is one of the big names in the United squad who's failing to meet expectations after he joined from Arsenal in January 2018.
Sanchez scored two goals in his first 12 Premier League appearances under Jose Mourinho, struggling to hit the same heights he experienced at the Emirates Stadium in terms of product in front of goal. He has made 12 appearances in all competitions this season, scoring just once.
The forward has done a good job of maintaining his fitness in recent seasons and has only been forced to miss three games through injury in the last two campaigns, per Transfermarkt.
The South American speedster shoulders a particularly heavy responsibility to impress for United, considering they made him the Premier League's highest-paid player.
A book released by Der Spiegel, titled Football Leaks: Uncovering The Dirty Deals Behind the Beautiful Game (h/t Telegraph's Sam Wallace), said Sanchez earns a basic salary of £391,000 per week at United, as well as a £75,000 bonus for every game he starts.
United play eight times in December, including against Sanchez's former team, Arsenal, on December 5 and against Liverpool 11 days later.
Sanchez may not have been playing close to his best this season, but it will do the team no favours if he's out for much of this busy period.