Bailly has missed United’s past eight matches across all competitions while undergoing “conservative” treatment.
But Mourinho seemed to suggest after Wednesday’s 1-0 win over Bournemouth that Bailly could face an even longer spell on the sidelines.
“I think it’s serious. We are trying conservative treatment, but if the conservative treatment is not working, he might have a surgery procedure,” the Portuguese told reporters, including The Telegraph’s James Ducker.
A £30-million signing from Villarreal last year, Bailly was supposed to create a foundation at the back. Injuries have instead limited the Ivory Coast international to 33 Premier League appearances out of a possible 55.