Rafael is being tipped as a future star by United manager Sir Alex Ferguson.
And Valencia told manutd.com: "It's incredible to see how much Rafa has improved this season.
"He has always been a great talent but this season you've seen a big improvement in his level of concentration in every game. It's great to see. If he continues at this rate, it won't be long before he is one of, if not the best right-back in the world."