The Belgium international has taken the time to praise the prolific striker but warns there is still plenty to play for in their Champions League tie with AC Milan
The Brazil-born attacker netted the only goal of the game to give his side a one-goal lead in the round of 16 tie and Courtois had nothing but praise for the prolific forward following the match at San Siro.
"Diego Costa is really important for us, he scores a lot of goals and it's massive that he scored on Wednesday," Courtois told the official Uefa website.
"It was a corner that a Milan player touched on and he was at the back post, as always, to execute a really good header."
The Belgium international went on to insist that the tie is by no means over and stressed that Atleti must be at their best in the return leg in order to seal qualification for the quarter-finals.
"It was a nice victory here, but it's not over yet. We have to be careful and play like we did today. We cannot say the tie is finished because it will be another hard game.
"They will come to Madrid trying to push hard to score a goal. We have to try to keep another clean sheet in order to qualify for the quarter-finals."