Raphael Varane sustained another injury during the first half of Manchester United's Champions League clash at Atalanta on Tuesday.
The France defender, starting for the second time since returning from a three-game lay-off with a groin issue, went down off the ball with 38 minutes played and was unable to continue.
Varane walked straight down the tunnel with members of United's medical staff and will now be considered a doubt for Saturday's derby with Manchester City.
United were already without Victor Lindelof in Bergamo after the centre-back suffered a minor injury in training this week.
Eric Bailly was handed just a second start of the season in the Group F contest alongside Varane and Harry Maguire, the other coming against West Ham in the EFL Cup on September 22.