Harry Kane has suffered a tear in his left hamstring, but Tottenham have given no indication how long the striker will be out for.
The England captain will definitely miss Sunday's FA Cup trip to Middlesbrough while the club's medical staff "continue to review the injury with treatment ongoing".
Kane, who limped out of Spurs' defeat at Southampton on New Year's Day, tweeted on Friday afternoon: "Head up. Tough times don't last, tough people do."
The loss of Kane will undoubtedly be a big blow for Spurs, who are just beginning to stutter under Mourinho after an initial bounce following his appointment.