The 28-year-old sat out of Spurs’ 7-2 walloping of Maccabi Haifa in the Europa League on Thursday night after being substituted at half-time of the Premier League clash with Newcastle last weekend.
It is a huge match for Tottenham on Sunday and Mourinho is desperate to have Son fit for it with the South Korean scoring five goals in his last three games.
However, the Portuguese manager is still in the dark as to whether Son will be available or not, with a late fitness test needed.
‘I don’t want to lie to you and say he doesn’t play and Sunday he’s there,’ Mourinho said. ‘At the same time I don’t want to say he plays and then he doesn’t.
‘So let’s wait and see, in this moment, I honestly don’t know.’
After the trip to Old Trafford there is an international break and an easing of their schedule, which Mourinho is delighted about.
‘I am used to it. It’s the last one, the last match,’ he said. ‘I don’t want to moan. I moaned enough and I moaned with real things on my hands and I don’t think anyone disagreed with me, even when I was moaning about the situation.
‘But now is only one more match, only the United match. But no excuses. They had only one match for the week and it was yesterday and probably the majority of the players who played are not going to play Sunday, but I want to forget that. We go there and we go there to discuss the game.’
Gareth Bale is not ready to make his second Spurs debut against Manchester United as he recovers from a knee injury, but should be fit by the time the Premier League returns after the international break.