Frenchman Yoro was substituted early in a pre-season friendly against Arsenal last week in Los Angeles along with forward Rasmus Hojlund, who will be out for six weeks with a hamstring injury.
"Yoro's rehabilitation starts now and we look forward to the 18-year-old returning to fitness in around three months," United said in a statement.
United signed Frenchman Yoro from Ligue 1 side Lille on a five-year contract in July.