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.
Kane has scored 27 goals in 31 appearances for club and country this season, but Mourinho is not prepared to dwell on it.
"I think we're going to lose him for some period and because I am totally convinced of that I can anticipate that," Mourinho said.
"I don't want to sit here with you crying in every press conference now.
"Everybody knows who he is, what he is, what he means for the team, the fans, the club.
"His quality, the routines that the team has playing with him, every minute of every game he doesn't play we miss, so every match that he doesn't play we're going to miss him.
"But I don't want to be crying here all the time, I want to focus now on the players that are available.
"Now Son (Heung-min) is back I prefer to speak about Son than Harry."
Son will be available at Middlesbrough and is likely to lead the line, with Lucas Moura or 17-year-old Troy Parrott also possible options.
While he could feature at the Riverside Stadium, Mourinho does not see Parrott as an immediate replacement for Kane.
"I think too soon. I think one thing is potential and another thing is conditions to express potential and I think he needs time," Mourinho said.
Australian Associated Press