Manchester United boss Michael Carrick said Carlos Baleba's injury "doesn't look great" after the midfielder was forced off during Tuesday's 3-2 win over Roma.
Baleba appeared to be in tears after pulling up holding his hamstring only moments into the second half at Old Trafford.
The 22-year-old was consoled by Carrick as he left the pitch and United's head coach hinted that he's expecting a long lay-off for the £70 million ($94m) summer signing.
"I haven't got any more news on Carlos at this stage," said Carrick.
"It's his hamstring, so we'll just have to assess what that looks like. Initially, it doesn't look great. Timelines, I haven't got much more to give you."
Baleba was making just his fifth United start after his debut was delayed by an ankle picked up during his preseason with Brighton.
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The Cameroon international is almost certain to miss Aston Villa's visit to Old Trafford on Sunday and is set to stay at Carrington during the international break for further assessment by United's medical staff.
Patrick Dorgu could feature against Villa after returning to training on Monday following a hamstring problem.
