The Spurs boss didn't react well to being told his star striker could be out for three months with a ruptured hamstring after he got hurt back in January during the 1-0 defeat at Southampton on New Year's Day (01.01.20).
In new Amazon Prime Video documentary 'All Or Nothing', Mourinho wasn't happy after head medic Geoff Scott revealed the severity of the injury.
In the footage, he can be seen running over to one of his coaching staff and saying: "Three months minimum... Minimum.
"Three months if he doesn't have surgery. Three months out. We're f*****."
In the end, Kane underwent surgery on the ruptured tendon in his hamstring.
He didn't return to action for Spurs until the season resumed in June after it was halted earlier in the year due to the coronavirus pandemic.
Earlier in the year, Mourinho claimed the England captain might not be held in such high regard if he decided to move to another club.
He said: "I think everyone thinks he's a fantastic player and Tottenham is so lucky to have him because he's the player, the person and the Tottenham boy.
"All this together makes him really a special player for us that probably wouldn't be so special playing for another team. He's really special for us, he's really special for Tottenham.
"Of course we want him to be happy and for him to be happy he wants victories, score goals, and I'm so happy after an incredibly difficult injury he's coming in the direction where he's going to end the season perfect.
"Hopefully the beginning of next season he will be even better."