Don't think the WC changes the perception of established players like Costa. People know who he is, he's scored 22 goals in 28 Liga matches and I think he has 30 in all comps so far this season. He's not flying under the radar, even last season when he was second fiddle to Falcao Liverpool were looking at him as a Suarez replacement.
It's Arsenal v Chelsea v staying at Atletico IMO, and that dynamic doesn't change much with the World Cup.