Manchester United defender Marcos Rojo is facing a period on the sidelines after dislocating his shoulder in training.
The incident happened on Wednesday and is the same shoulder Rojo dislocated against Manchester City last season.
On that occasion Rojo, 25, did not have surgery but still missed four weeks.
United are refusing to comment but manager Louis van Gaal is likely to discuss the extent of the injury at his weekly media briefing on Friday.
Rojo, a £16m signing from Sporting Lisbon in 2014, has made 11 appearances for United this season, his latest coming in the Champions League draw with PSV Eindhoven at Old Trafford on 25 November.
The Argentine joins fellow left-back Luke Shaw on the sidelines.
Shaw is still a long way from a return after suffering a double leg-break during the away game against PSV in September.