The 21-year-old was given his Man United debut by David Moyes in 2013 and will be reunited with the Scottish boss at the Stadium of light this campaign.
Januzaj spent part of last season on loan at Borussia Dortmund, but returned back to Old Trafford after failing to cement himself in the first-team.
He will be available to make his debut against Manchester City in the Premier League this weekend and Mourinho believes he is in the right place under Moyes.
“He fits in my plans which is why he’s on loan at a Premier League club,” he said.
“If he didn’t fit into my plans then we don’t care about where he’s going. We care about him and think he’s a talented player.
“I want to give give him all the conditions to develop and to play well. He is in the right place with a manager that likes him a lot. Hopefully everything will go well for him because we want him back!”
It’s a brilliant move for Januzaj to pick up some experience in England which he would not have been able to receive under Jose Mourinho at Man United.
Being able to play on the first-team at Sunderland each weak will help add consistency to his game and will drastically help his development.
Moyes coached Januzaj through his impressive breakthrough season, and although the Belgian hasn’t been able to replicate his form, hopes are once he is reunited with his former manger he’ll be able to pick up from where he stopped.