The Ivorian picked up a groin injury against Chelsea in a 1-0 away defeat to the Premier League champions. He’s missed eight games since then, and could miss more a further 20 games.
Bailly has been undergoing conservative treatment since picking up the injury, but Mourinho told reporters ahead of Sunday’s Premier League clash against West Bromwich Albion that the 22-year-old has been forced to undergo surgery.
Mourinho told MUTV that Bailly “goes to surgery, the decision is made.”
“I don’t say [that he’s out] for the rest of the season but it’s for the next two or three months.”
Bailly cemented himself has as Man United’s first choice centre-back quickly after arriving for around £30m from Villarreal last summer as one of Mourinho’s first signings at he club.
He was a surprising addition to Mourinho’s side after the Portuguese took the reigns at Old Trafford from Louis van Gaal, but he has excelled.
Bailly was forming a solid partnership with Phil Jones in the opening two months of the season before the pair both picked up injuries, though Bailly’s is far more serious after Jones returned for the Reds against AFC Bournemouth in Wednesday night’s 1-0 win.