Bailly has not appeared for Manchester United since their 1-0 defeat to Chelsea at Stamford Bridge in October.
The Ivory Coast defender was expected to return in mid-November but his comeback was delayed.
And speaking after United’s narrow 1-0 win over Bournemouth, Mourinho described Bailly’s injury as “serious”.
He then added that an operation was an option if “conservative treatment” didn’t work.
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