It does no one a service to be carried away with optimism. Zaha has the potential to nail that right wing but he'll have to earn as from the start (and specially the tough opening 5-6 fixtures) Moyes will probably trust more experienced wingers.
He'll have to earn his place, of course, but if Valencia doesn't improve his game then imo Zaha will quickly step up. It's not a case of getting unduly carried away with optimism; simply the recognition of a young player of great talent and potential being ready to make his mark on the team. Zaha can play from either wing remember, so he's not just competing with Valencia but with Nani and Young also, all three of which have big question marks over their heads in the squad right now for various reasons. It's perfectly reasonable to be optimistic about Zaha making an immediate positive impact on our season. We're not talking about some youth player with barely any senior experience to his name; Zaha has played 142 senior level games over the last three seasons, won Championship player of the year whilst playing a huge hand in securing promotion for Crystal Palace and cost us £15m.