The Belgian made a blistering start to his Manchester United career under David Moyes, only to then encounter a miserable two years under Louis van Gaal before being sent on an unsuccessful loan spell at Sunderland last season.
Januzaj completed a move to Sociedad last month, signing for a fee within the region of £9.8, and interestingly has a buy-back clause on his contract.
And the 22-year-old, when asked about Mourinho, insisted that he would prove his former boss wrong.
“Obviously, he is a great trainer but I did not have much of a relationship with him,” he said.
“I respect him a lot because he is the one who left the door open for me to leave Manchester United. What I want now is to prove that I am a good player and that they were wrong to let me go.”
Januzaj clearly feels hurt by the way his United career, so promising to start with, ended in such a dismay following a long, painful descent into anonymity.
But he should realise that succeeding at his new club is not tantamount with proving Mourinho wrong: to do that, he would have needed to step up to the plate at Old Trafford when called upon – something he failed to do for almost four years.
The winger was sold because he wasn’t good enough for Man United, and no kind of success in Spain will change that notion.