Manchester United's world-record signing Paul Pogba was forced off with an apparent hamstring injury during his side's Europa League last 16 game at home to Rostov on Thursday.
The France midfielder, criticised for a poor performance in Monday's 1-0 FA Cup quarter-final defeat at Chelsea, pulled up early in the second half holding the back of one of his legs.
He hobbled off the pitch and walked straight down the Old Trafford touchline to the tunnel, with Marouane Fellaini taking his place.
The score was 0-0 at the time, with United holding the advantage of an away goal from last week's first leg, which ended 1-1.
An injury to Pogba, who turned 24 on Wednesday, would add to manager Jose Mourinho's problems in midfield, with Ander Herrera facing a two-game domestic suspension following his dismissal at Chelsea.