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If anyone was in any doubt why Chelsea had bought Diego Costa in the summer, this game should have erased them. Costa was a constant snarling, irritating, brilliant presence—annoying the Everton defence with his confrontational style and then putting them to the sword with his clinical touch in front of goal.
Presented with a glorious chance after barely half-a-minute, Costa was cool and collected enough to make the most of the gift and put it away. From there, Chelsea's evening should have been a straightforward one—that it was not had little to do with Costa.
As the game wore, Costa continued to be a threat in the final third, although his second (and final) goal only came after the game had already been won. But he showed his mastery of another side of the game, the psychological one, as he visibly riled his opponents. Whether it worked or not is hard to gauge conclusively, but the fact Everton went on to concede six times suggests they were not at their most concentrated.
Costa now has four goals from three league games, and Chelsea have nine points from a possible nine points.
As Mourinho told reporters (per London Evening Standard):
He's a good finisher, he gives us not just that final touch but he also operates defensively and I can remember him making some tackles on the edge of the box so he is giving us what we expected.
With Fernando Torres gone, Mourinho also confirmed (per the Daily Star) after the match that Chelsea are looking to add QPR's Loic Remy to the squad. The Frenchman already should know what his role is going to be, however—like Didier Drogba, he is simply going to be back-up for the main man Costa.
After just four games, the naturalised Spaniard's supremacy is already assured.