As far as I know his right arm and side of his body is still made of zetsu goo. Also. by no means is the zetsu goo weak. Zetsu goo is extremely powerful. Strong enough to withstand a direct blow from suigetsu's sword with ease. So it isn't surprising that it is able to withstand a rasengan. Also I believe the zetsu goo has the ability to match Obito's skin color or can be painted to match it at the very least.