The promising full-back will spend a season away from Old Trafford and Sir Alex Ferguson has already suggested a stint in Portugal is on the cards.
The Brazilian siblings are inseparable off the field but the manager feels Fabio will benefit from a new experience after being unable to dislodge Patrice Evra from the team.
"It's so, so hard," he told MUTV about the potential move. "It's very, very difficult for me and my brother as well but this is life.
"I have to play more games to improve my football and it's why I think it's better for me and for him as well.
"I had a good start and played a lot of games in the Champions League but, at the end, I've not played in many games. But it's okay and we go to the next season."