The England captain was withdrawn at half-time of the chastening 3-1 home defeat to Liverpool after suffering knocks to both ankles in the first half, but it was the one to his left joint that seemed most damaging.
Kane has a long history of suffering ligament damage to his ankles and Mourinho was in cautious mood immediately after the game against the defending champions, who won through goals from Roberto Firmino, Trent Alexander-Arnold and Sadio Mane.
“For Harry to leave a game, when he wants to play always and with the team losing, for Harry to come out is not a nothing injury,” Mourinho said. “It’s an injury. For how long I can’t tell you, I don’t know. We have to wait and see.”
It capped a pretty miserable night for Spurs, who saw any faint hopes of a title challenge killed off by a Liverpool side who were back to their best.
Spurs were architects of their own downfall, though, as defensive mistakes gifted Liverpool all three goals and Mourinho was not shy in highlighting the matter.
“It’s a performance totally affected by defensive individual mistakes,” he said. “The team was very solid, very confident and in the first half we made three defensive mistakes – individual.”
Asked whether he thought the individuals were not good enough, he added: “I can’t say that. I’m not going to say that. These are your words, not my words.
“One thing is mistakes and another thing is not to be good enough. That’s a different story. I don’t say that at all. Everybody has to assume their responsibilities and try to improve.
“The reality is that you don’t need me to comment because you saw the game and you can give many directions to your analysis. You always end in the goals because the goals were the crucial parts of the game.
“By analysing the goals you can see that the three goals we conceded, the goals that was disallowed by Firmino and the Mane chance in the first minute.”
Spurs will be back in action at lowly Brighton this Sunday. Graham Potter's men are 17th in the table and five points above the relegation places.
Albion also have a few injury issues of their own and will assess Arron Connolly (hamstring), Adam Lallana (groin), Danny Welbeck (knee) and Alireza Jahanbakhsh (hamstring).