The 20-year-olds may have been in England for more than two years - and have known the United boss twice as long - but the pair are still easily mixed up.
"When we came to the club, Fabio had just got married and wore a ring," Rafael told Globoesporte. "So our boss just had to look at the left hand to tell us apart. It was quite funny.
"But now we are both married and wear rings. This has made his life more difficult and occasionally he still gets it wrong. We laugh, but he takes it well."