The South African, who trained with the 18-year-old last season in the reserves, had been at Old Trafford since 1999 when the Portuguese star arrived from Sporting in 2003.
And now he sees the same self-belief in the Belgium-born teenager as he did in Ronaldo a decade ago, and is convinced Januzaj can have a big future.
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"He has so much belief and confidence in his own ability and that's hard to see in young players these days. He reminds me of Ronaldo when he arrived.
"He believes in himself and that takes you far in this game. He has great talent and he is a level-headed guy.
"He is doing incredibly well, he is already one of the key players in the team and hopefully that will continue."
Fortnue believes it is Januzaj's self-belief that sets him apart from his peers and has backed the youngster to continue his rapid development.
"If Adnan can follow what Ronaldo did it would be unbelievable for himself and the club," he continued. "Adnan has exactly the same mentality [as Ronaldo].
"I don't think Ronaldo lacks confidence, and that is why he is one of the best players in the world. Adnan still has a long way to go, he has had a fantastic start though and I wish him the best.
"The self-belief and confidence set him apart from the other young players I saw at United a couple of years ago and even though he is playing at a higher level now, he has the same mentality.
"He is a fantastic player and there is more to come from him. Once he gets a few more games under his belt you will see an even better performance from him."