We've seen quality players, including quality diminutive players from the Iberian peninsula at Arsenal. And Vieira's no Cazorla.
No need to be mean about it, it's facts isn't it? At present when Vieira comes on he doesn't influence our matches, doesn't find dazzling moments, looks a bit lost, makes basic errors, loses the ball, and looks scared about opposition players who weigh 20 kilos more than him mowing him down.
If he's supposed to be Øde's number two in the pecking order—and the Norwegian does sometimes disappear in highly physical matches, but not like Vieira—then you'd expect Vieira to be able to do some of (or any of) the same sorts of things, but he doesn't seem to.
All this can be normal for players adapting to the league's relative intensity, but Vieira actually has to adapt before we know he can be useful. There's no assumption to be made that it's even possible. If he does it, great.
