Costa followed up his brace for Spain last Monday with another two goals at the Liberty Stadium at the weekend, the second of which helped the Blues salvage a point in a game they had dominated.
"His work [in training] is reflected in the games. He is scoring a lot of goals for us. He works very hard for the team, he makes a lot of movement and he is a big part of our team," the full-back told Chelsea's official website.
"We are happy he is scoring goals, because if he is scoring goals it seems we are doing the right thing and he is getting the chance to score."