The Brazilian right back returned from the summer break a married man, taking away the Ferguson's ability to tell apart Rafael from his identical twin brother Fabio.
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.
Lookalikes: Rafael (left) and his brother Fabio (right), who started against Rangers
'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.'
Fabio was handed a rare start at left back in Tuesday's goalless Champions League draw with Rangers, while suspended Rafael watched from the stands.
Both were distraught to see Antonio Valencia suffer a freak ankle injury that could keep the winger out for the rest of the season.
Fabio said: 'I was very sad about his injury, he is one of mine and Rafael's best friends at the club, and an excellent player.
'I will go with my brother to visit him at the hospital, but I wish him a speedy recovery. He's a great guy, fun and always helpful.'
Manchester United star Rio Ferdinand believes he will need five more games to reach his peak Manchester United boss Sir Alex Ferguson makes 9am dash to crocked Antonio Valencia's hospital bedside MANCHESTER UNITED FC