Januzaj scored twice on his full Premier League debut against Sunderland and has featured on eight occasions in total for United this season.
The 18-year-old has made a positive impression on the former South African international who has witnessed the starlet's development first-hand after his spell with United's reserves while completing his coaching badges.
"Everyone around the club knew that he was special," Fortune said.
"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 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 was making so much progress and when he went to train with the first team, some of those lads were talking about him.
"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."
United tied Januzaj down to a five-year contract earlier this month and Fortune is optimistic the teenager can follow in the footsteps of former Old Trafford hero Ronaldo.
"If Adnan can follow what Ronaldo did it would be unbelievable for himself and the club," he said.
"Adnan has exactly the same mentality. 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.
"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."