Sanchez started the match but was forced off after 52 minutes after colliding with Jan Bednarek.
Chile's national team physio told El Mercurio (h/t MailOnline's Jack Gaughan):
"We spoke to the player, and he had scans on the injury (on Sunday).
"He will be available for the Copa America, but he will be unavailable to play for between six and eight weeks.
"The medial ligament strain is an annoying injury but not such a complex one."
One more setback is further bad news for Sanchez, who has endured a torrid time since swapping Arsenal for United back in January 2018. A spate of fitness problems, combined with indifferent form, have rendered the gifted attacker a flop at Old Trafford.
He's United's highest earner, but Sanchez has been left struggling for playing time while Anthony Martial, Marcus Rashford and Jesse Lingard have all been preferred. Caretaker manager Ole Gunnar Solskjaer has even opted to deploy full-back Diogo Dalot on the right side of a front three at times.
Such a move only further highlights how far the stock has fallen for Sanchez. He appears to have lost some of the acceleration and strength that helped make him prolific for the Gunners.
Wear and tear seems to have diminished a player who had already missed nine matches earlier in the campaign with a hamstring problem.
Now United must decide if this latest injury is the ideal time to move on from Sanchez altogether. If others thrive in his absence, the club may give up waiting on Sanchez to come good and instead part company with a player who has rarely looked comfortable since donning a United shirt.