The lookalike pair create regular confusion at Old Trafford, often deliberate, and have even confused match officials during their time in England.
At Barnsley last season, referee Chris Foy booked Fabio instead of Rafael so confused did he get by the pair's presence, so it is easy to see how Ferguson might get away with fielding one instead of the other.
"I could play Rafa and say he was Fabio quite easily. There is no way they could know," he laughed.
"Probably they have the same DNA. Maybe it wasn't Rafael who was sent off last week? It is definitely a thought.
"The referee booked the wrong one at Barnsley, so one day you could play one in the first-half and the other in the second."
Ferguson's joviality betrayed a tough decision for the United manager, with Fabio, John O'Shea and Wes Brown all in contention to face Birmingham at the start of a crucial four-day period that ends with a trip to Blackpool, which Rafael will return for after being handed merely an #8,000 fine and a warning for his remonstrations to Mike Dean at White Hart Lane.
Back-to-back victories would leave the Red Devils in pole position to regain their title, registering a record 19th championship in the process.
Not that Ferguson can afford to take anything for granted against a team that snatched two points off United thanks to Lee Bowyer's last-gasp equaliser at St Andrew's on December 28 and for whom the phrase "hard to beat" could have been invented.
"We are expecting the same battling virtues from Birmingham that we saw at St Andrew's," said Ferguson.
"They do have fighting qualities in their team. They will play five across the middle of the pitch, will keep possession and try to make it difficult for us.
"But at Manchester United, when you play against teams who have a certain way of playing, you have to find solutions."
Birmingham manager Alex McLeish believes United can emulate Arsenal and go through the season unbeaten in the Barclays Premier League.
McLeish believes it is "ominous" that United have still to taste defeat despite not firing on all cylinders during the early part of the campaign.
He also feels the experience of his mentor at Aberdeen, Sir Alex Ferguson, will again be crucial and give United the upper hand during the run-in.
McLeish said: "I think it's ominous, the fact that they've got to this stage unbeaten and their form is probably gathering momentum.
"They are probably looking better now than in the earlier part of the season when they had a lot of draws.
"It's amazing that they are unbeaten and I think that they can only get better."
Asked if they could go throughout the league campaign without losing, McLeish added: "Of course they can. It would be a hell of an achievement.
"In terms of the title, it is such a tight league but Manchester United have so much experience of this situation.
"There is no man who has got greater experience than Sir Alex when it comes to the run-in, and the run of games after January."
McLeish has challenged his players to relish the task facing them against the Red Devils.
He said: "The spirit in the dressing room has never been in doubt. Players have always gone right to the very end of games.
"We know Old Trafford is a very difficult place to go and win but it is a great challenge for the players.
"It is a fantastic arena and the pressure is off us, in that we won't be favourites to win the game.
"But it is not worth getting out of bed in the morning if you don't think you can go and do something.
"We've got to go there and believe we can surprise them, surprise everyone.
"It is a privilege for the boys to be playing professional football and it should be their ambition to go to places like Old Trafford and show how well they can do."