Aaron Wan-Bissaka is set to be ruled out for up to two months, talkSPORT understands.
The defender injured his hamstring after coming on as substitute during Saturday's 3-1 defeat at home to Brighton.
Now, the Red Devils have confirmed in a statement on their website that the right-back faces 'a period on the sidelines.'
Wan-Bissaka's setback poses as yet another blow to Man United's defensive options.
Centre-back Raphael Varane missed the defeats to Arsenal and Brighton due to injury whilst left-backs Luke Shaw and Tyrell Malacia have also been sidelined.
Tom Heaton, Kobbie Mainoo, Sofyan Amrabat, Mason Mount and Amad Diallo are all out, too, whilst Jadon Sancho and Antony are unavailable due to off-field issues.
Diogo Dalot started at right-back against the Seagulls, with Wan-Bissaka coming on to replace him late on.
But the 25-year-old had started all four fixtures in that position before Saturday, providing the assist for the winning goal against Wolves on the opening weekend.
Wan-Bissaka will miss Man United's Champions League opener against Bayern Munich, as well as the upcoming Premier League fixtures against Burnley, Crystal Palace and Brentford.
He'll also be absent for the Carabao Cup tie against Palace and their Champions League group game against Galatasaray.
Man United's statement on Wan-Bissaka's injury read: «Aaron Wan-Bissaka is set for a period on the sidelines after sustaining an injury during the closing stages of Saturday’s game against Brighton & Hove Albion.
»The right-back came on as an 85th-minute substitute, having started the game on the bench after battling illness during the week.
«Further assessment will be needed to determine how long Wan-Bissaka will be out for, but initial indications suggest itRead on talksport.com