Mourinho revealed the update while speaking to MUTV:
The Ivorian only arrived last year but established himself as United's best centre-back with a series of assured and committed defensive performances, and he perhaps grew even more impressive as the campaign went on.
Fortunately for United, Marcos Rojo recently came back from a long-term knee injury, while Phil Jones and Victor Lindelof have performed well in recent matches, per football writer Liam Canning:
Bailly's injury is nevertheless a blow to manager Jose Mourinho.
As the Red Devils seek to close down an 11-point gap in the Premier League and set their sights on competing in the latter stages of the UEFA Champions League, it's imperative they are able to name strong teams in each game.
Bailly is a key player, so with him out injured for a significant part of the season, his loss will be keenly felt if those covering for him suffer a dip in form or face fitness issues of their own.