United may have had a difficult start to the season, but Januzaj has hit the ground running in his first campaign with the senior squad.
The 18-year-old scored twice on his first start, against Sunderland, and has featured eight times for the Barclays Premier League champions this season following his promotion from the reserves.
Fortune trained with United's second string last year while he was taking his training badges and he identified Januzaj's potential straight away.
The South African immediately saw similarities to his former United team-mate Ronaldo in the teenager, spotting the talent and confidence to take on some of the best players around.
Fortune, who is supporting the Manchester United Foundation Chevrolet Trax eBay auction, said: "Everyone around the club knew that he was special.
"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 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."
Fortune was four years into a seven-year stint at United when Ronaldo walked through the door at Carrington to train with his new team-mates for the first time.
Sir Alex Ferguson, who described Ronaldo as the most gifted player he had worked with, molded the Portuguese into the superstar who moved on to be a sensation for Real Madrid.
Fortune knows Januzaj has a long way to go before he can match the achievements of Ronaldo, but he thinks the midfielder is on the right track.
"If Adnan can follow what Ronaldo did it would be unbelievable for himself and the club," Fortune said.
"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."
Januzaj's early performances have been so impressive that Roy Hodgson is monitoring the player in the hope he may one day pledge his allegiance to England.
Januzaj, who is eligible to play for several nations, put on another sterling display on Tuesday night, winning a penalty in United's 4-0 win over Norwich in the Capital One Cup.
The victory means David Moyes' men are now unbeaten in six matches, and Fortune sees reason for optimism after such a difficult beginning to the Scot's reign.
"It has been a tough start," said Fortune, who played 126 times for the Red Devils.
"But I am sure the other teams will drop points. There is a lot of confidence within the team and now hopefully we can go on a good run and by Christmas or January we are up there not far behind (leaders) Arsenal."