Sir Alex believes Rafael can achieve the same legendary status as Gary Neville.
The Brazilian has once again earned praise for his development after emerging as one of the club's best performers this season. Critics may have pointed out flaws in his game as a youngster but, now 22, the right-back is looking a world-class talent.
"He's a Brazilian who trains like a beast," said Sir Alex. "He never misses training - the boy is just a football fanatic. When he goes home in the summer, he plays every day for his street team back in Brazil.
"Maybe he had rashness and the impetuosity of a young boy but somewhere along the line that maturity comes along and the rashness disappears. His form this season has been brilliant and the boy is really tough now."
Sir Alex was asked if he can see Rafael going on to enjoy a career similar to Gary Neville's distinguished stint. "Definitely," he replied. "There's no doubt about that. Gary was not the finished article when he came into the team but was lucky in the sense that he came in at a time when we were trying to push forward all our young players. But Gary improved and became absolutely fantastic - probably the best right-back the club has ever had.
"It did take time for him in terms of his use of the ball and crossing but all these things developed because he wanted to work on them. Rafael is definitely the more naturally talented. Gary came to us as a centre-back, don't forget."
Rafael showed no ill effects from playing the full 120 minutes at Chelsea in midweek when producing another accomplished display in the 2-1 win over Arsenal.