Manchester United defender Marcos Rojo has had surgery on the shoulder he dislocated in training last week.
The decision to operate was taken in an effort to prevent the problem recurring as Rojo, 25, suffered the same injury against Manchester City last season.
He did not have surgery on that occasion and was out for four weeks.
The Red Devils will not say how long the Argentina international will be out for, but a source close to the player expects it to be about four weeks.
United boss Louis van Gaal is short of defensive options after Chris Smalling and Matteo Darmian were injured during Tuesday's 3-2 defeat at Wolfsburg.
Central defender Smalling (groin) and full-back Darmian (hamstring) are both doubts for Saturday's Premier League trip to Bournemouth.
Full-backs Luke Shaw (leg) and Antonio Valencia (foot) have long-term injuries, while the versatile Phil Jones missed Tuesday's match with an ankle problem.
Guillermo Varela, 22, made his first start for United in Wolfsburg, with 18-year-old left-back Cameron Borthwick-Jackson replacing Darmian as the Red Devils were knocked out of the Champions League.
Midfielders Ander Herrera and Morgan Schneiderlin, and forward Wayne Rooney are also sidelined.