Manchester - Manchester United manager Jose Mourinho said he did not know how seriously Paul Pogbahad injured himself after the France midfielder limped off during Tuesday's 3-0 win over Basel.
Making his debut as United captain, Pogba lasted just 18 minutes of the Champions League group game before being forced off by an apparent left hamstring injury, with Marouane Fellaini taking his place.
Pogba abruptly pulled up in midfield, clutching the back of his left leg, and after being helped off the pitch by a United physio he showed his disgust by tossing his armband aside as he hobbled down the touchline.
"I don't know obviously, but by experience, just by looking and feeling, it's a hamstring," Mourinho told reporters at Old Trafford.
"Big, small, I don't know."
Fellaini, Romelu Lukaku and substitute Marcus Rashford scored as United marked their return to the competition with a straightforward win.