Manchester United manager Jose Mourinho said Paul Pogba was facing around three weeks out after he sustained a hamstring injury during Thursday's 1-0 Europa League win over FC Rostov.
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.
Asked if Pogba would be out for around three weeks, Mourinho replied: "Yeah. I don't know for sure, but for sure, no Middlesbrough (on Sunday) and no national team."
The score was 0-0 at the time of Pogba's exit and United went on to win 1-0 courtesy of Juan Mata's 70th-minute goal, completing a 2-1 aggregate victory that sent them into the quarter-finals.
The injury to Pogba, who turned 24 on Wednesday, adds to Mourinho's problems in midfield, with Ander Herrera facing a two-game domestic suspension following his dismissal at Chelsea.