Ndombele arrived for a club record £54million last year but struggled to make an impact, with injuries limiting him to 17 starts and Mourinho singling him out for criticism before the Covid-19 shutdown.
However, the 23-year-old has since shown he is focused and applying himself, Mourinho said.
“Last season, he was stuck in a situation where I couldn’t see evolution,” Mourinho told reporters. “In this moment, he’s training very, very well.
“In this moment, I believe in Tanguy. I never doubted his quality. Never.
“I doubted in some moments his motivation and commitment and professional attitude.
“In this moment I think that Tanguy can do well for us.”
Spurs, who lost their Premier League opener 1-0 to Everton, travel to Southampton on Sunday.