Son has been out since pulling up with a hamstring injury 18 minutes into last month’s north London derby defeat to Arsenal.
He did not feature for South Korea during the international break but returned to full training with Spurs on Friday and is available to face Newcastle at St James’ Park.
“Good, Sonny’s good,” said Mourinho when asked about Son’s fitness.
Pushed on whether Son is ready to face Newcastle, Mourinho said: “Yeah. He will be selected.
“He was injured, he trained with the team fully first time this morning. Yesterday he was coming to do partial exercises, not a complete session.
“This morning was the first day he did it and last week he did not train at all so it was completely impossible for him to play international, even if the game was yesterday or Tuesday. So this time was a good time for him to recover from injury.”
Mourinho also confirmed Sergio Reguilon is fit to face Newcastle but Spurs will be without Ben Davies and Matt Doherty for the trip to the north east.