Romelu Lukaku was stretchered off the field in the first half of Manchester United's 0-0 draw against Southampton.
Lukaku clashed heads with Wesley Hoedt.
Jose Mourinho told the BBC after the game that Lukaku would be "out of football for a while."
Lukaku was attended to by United's medical staff on the pitch for around six minutes before leaving on a stretcher wearing an oxygen mask.