Matteo Darmian, Manchester United right-back, will be out for "several weeks" after dislocating his left shoulder in Saturday's 2-1 defeat at Sunderland, manager Louis van Gaal said.
Italy international Darmian, 26, was replaced by young United defender Donald Love in the 37th minute after landing awkwardly following a collision with Sunderland goalscorer Wahbi Khazri.
"Unfortunately, Darmian is a dislocation and he's out for several weeks," said Van Gaal. "We don't have any more full-backs."
Darmian joins fellow specialist full-backs Luke Shaw and Guillermo Varela on the sidelines, while Antonio Valencia, Marcos Rojo, Ashley Young and Phil Jones, who can all play at full-back, are also currently injured.
Daley Blind and Chris Smalling, who have played at full-back in the past, are being used at centre-back.
Darmian joined United from Torino last year in a transfer reported to be worth £12.7 million ($18.4 million, 16.4 million euros).
United's defeat kept them six points below the Premier League top four, jeopardising their chances of qualifying for the Champions League.