Dzagoev's been on the verge of a move to a top European league for years now. He hasnt just appeared out of thin air. But it's not cheap to get players out of Russia. Especially those as 'hyped' as Dzagoev.
With his contract running down and him looking decent in this tournament though now's the perfect time.
Cant say I watch any Russian league matches so I cant speak of his quality, but he's long been mooted as a big talent. Not signing someone because they do well is a bit backwards no?