Manchester United manager Sir Alex Ferguson is tipping Rafael to become one of the world's best full-backs after the youngster signed a new four-year deal.

The 21-year-old has earned a call-up to Brazil's squad since joining United four years ago and Ferguson believes continued development will see the defender progress even further at the highest level.

"Rafael has all the attributes to become one of the best full-backs in the world and I am delighted he has signed a new contract," the Scot told

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