Luke Shaw looked to have picked up a knock after being substituted in the first-half of Manchester United's game against Brentford on Wednesday.
The United defender started at Old Trafford in his first game since penning a new four-year deal with the club, starting brightly and moving into midfield as Erik ten Hag's side took a first-half lead through Marcus Rashford.
But the 27-year-old received an early yellow and then seemed to be holding his hamstring with United hesitant to play the ball out to him.
He eventually came off on 37 minutes for Tyrell Malacia, exchanging brief words with Ten Hag on his way off before heading straight down the tunnel. LIVE: United vs Brentford latest from Old Trafford A potential injury would come at the worst time for Shaw, who has been ever-present in the United side.