The BBC are reporting that Diego Costa has not only undergone his Chelsea medical, but that he's passed the examination. If that's the case, his signing would be all but complete, and we should be hearing an announcement from the club in relatively short order. It's unlikely that Costa would be officially introduced before the completion of the World Cup, but the club have confirmed signings before they became official in the past.
Other outlets are picking up on this, but the fact that a more reputable source like the BBC is among the first to announce this news, there's a very real chance it's accurate. If so, it's awfully nice to see that our biggest need has already been filled, and that the club can begin to turn their attention toward the midfield and squad depth.
Assuming this is correct, it's also a good sign that Costa's hamstring is nothing too serious. I suppose that's good news if you're a Spain fan.