Defender Victor Lindelof needs a scan and is expected to be out for "quite a while", according to Manchester United manager Jose Mourinho.
Lindelof sustained a muscle injury on international duty with Sweden but played through the pain in Saturday's goalless draw at home to Crystal Palace in the Premier League.
The centre-back has been a regular starter for United this term, named in Mourinho's starting line-up for 11 Premier League matches and three Champions League games.
But the Portuguese will need to find an alternative for the 24-year-old as he is set to miss Tuesday's Champions League game at home to Young Boys and could be unavailable for a number of weeks.
"He will be out for quite a while," Mourinho told a news conference on Monday.
"An injury - a proper injury - that will keep him out for a while.
"If we have him for the Christmas period, when we need as many as possible, it will be good."
United are also without Argentina international Marcos Rojo for the visit of Young Boys.
"We have [Eric] Bailly and [Phil] Jones," Mourinho said.
"Marcos Rojo has some little injury, and Victor an injury, a proper injury.
"So Jones, Smalling and Bailly are the three we have."