We all forget that the da Silva twins are only 19-20 years old. At what age did the likes of Scholesey, Butt, Neville, Beckkham start for United. Only about 22-23 years of age. Both Fabio and Rafel will need 2-3 more years of experience- where they will undoubtdely make a few mistakes here and there. But I am sure they will develop to become first team United regulars within 3 years time.
Most RedCafe posters seem to compare Rafael (RB) and Fabio da Silva (LB) with what they saw with the great Brazilian fullbacks of the 80s, 90s,2000s such as Roberto Carlos, Cafu, Jorginho, Maicon, Branco... Please do not forget that even these great Brazilian internationals spend many, many years honing their craft to reach their peak as footballers.
Another comparison should be with Cristiano Ronaldo. How different was he at 19 years old (2nd season at United-called showboater, one trick poney) and when he was 23 years old (scoring shitloads of goals, winning World Player of the Year)? Totally different beasts. Give our da Silvas a few years time. If they develop to their maximum potential, we have gems on our hands.