Son is one of the most important members of Tottenham’s squad and is one of the first names on Mourinho’s team sheet when fit and available.
The South Korean hobbled off during the north London derby on Sunday after just 19 minutes due to a hamstring injury.
The Tottenham boss has ruled him out of their Europa League last 16 return leg against Dinamo Zagreb on Thursday night.
He said in a press conference: “I don’t know [when he will be back]. Definitely out for tomorrow.”
Son has been named in the South Korea squad for the upcoming international games, but Mourinho made it clear that the player will not be released if he is injured.
The Spurs manager insisted that he cannot play for his national team if he is not fit enough to play Tottenham at the moment.
“If a player is injured. It doesn’t matter if it’s a friendly or an official match.
“If we get to next week and he is injured and we can prove that to the Korean football association then what can they do?
“If he can’t play for the club, he can’t play for his country.”
Son has 89 international caps to his name for South Korea and has scored 26 times for his country.
