The Brazilian brothers have spent the past four years at Old Trafford and have made over 120 appearances between them.
However, Fabio's path to a regular slot at left-back is blocked by Patrice Evra, while Rafael has competed with the likes of Gary Neville and John O'Shea for the right-back berth in the past, and now Phil Jones and Chris Smalling.
The defensive duo hope to represent Brazil at the 2014 World Cup on home soil, but Rafael is happy to bide his time to become a fixture in the United team.
"One target is playing for Brazil at the World Cup and, for that, we need more games for United, but we're relaxed," Rafael told the Sunday Times.
"We're young and, here, maybe only Rooney can play every week, maybe Patrice. You have to understand it's part of being at a big club."
The 21-year-olds were both on the books of Fluminense prior to joining United in 2008 and Rafael acknowledges it will be difficult if one of them ends up playing for another club.
He added: "We have a special bond, we're best friends. When we were five years old someone said, 'Come on, let's play football', we start there and we've never split.
"But sometimes we think what it would be like. I know he could go to another club, one day.
"If it happens it will be hard but we're prepared, it's football, it's life."