The 18-year-olds joined United from Brazilian side Fluminese in the summer.
Rafael has made an instant impact at Old Trafford and has put Gary Neville and Wes Brown under high pressure for the right-back position.
“When a guy turns the corner you cannot do anything about it,” said Ferguson of Rafael.
“I feel the boy has something special.
“He has such character, and such talent, and great courage. He’s got that Brazilian mentality where he wants to get on the ball all the time.”
“I have absolutely no doubt at all that he will do the same for us as Rafael,” added Ferguson on Fabio.