He sustained the injury just minutes into the second-half after going down heavily trying to challenge Eden Hazard.
The Ivorian was seen clutching his knee before being helped off the pitch as he was replaced with Marcos Rojo.
Speaking after the game, Mourinho confirmed it was a bad injury for Bailly.
“I’m afraid it’s a bad injury. In the knee ligament area. It’s bad,” he said.
It’s a massive blow for Man United as he has been on of our standout performers of the season so far.
Many believed it would take time for Bailly to adapt to football in England, but he’s had an instant impact at Old Trafford, winning United’s Player of the Month for August.
He’s quickly cemented his place in the side and his presence will undoubtedly be missed as United are now stuck with Daley Blind and Chris Smalling at the centre of defence, who have been extremely shaky in recent showings.
Hopes are Bailly will be action soon, but after Mourinho’s confirmation, it’s not looking good for the Ivorian.