The former Atletico Madrid man continued his goalscoring streak against Aston Villa despite ongoing fitness problems.
Despite struggling in his native Brazil during this summer's World Cup while on international duty with Spain, Costa has taken to the English top flight with consummate ease, scoring 8 goals in his opening 6 games, and playing a huge part in Chelsea's early stint at the top of the standings.
Yet Jose Mourinho has revealed the 25-year-old has kept up this form despite barely even training with his new teammates since his switch from La Liga.
At the end of Atletico's 2013-14 season, Costa experienced issues with his hamstrings that saw him play a bit part in the club's final few La Liga games as well as the Champions League final against Real Madrid.
And it appears that hamstring niggle has carried over into Costa's debut campaign in the Premier League - not that anyone would notice.
"He's doing almost nothing in training," said Mourinho. "He's just resting and recovering from the tight muscle he has."
"He cannot be on top of his game.
"I know a striker scoring goals is always nice for him and for his confidence, especially for a player like him who is not training as he should, because we are protecting him in certain situations."
The Portuguese earlier told BBC Sport that the Spaniard's influence on the game with Aston Villa, which saw him score and assist for the Blues, was huge, even if it came as a result of Chelsea playing so well.
"It's a gamble to play him in a second consecutive game without at least a week of recuperation.
"The team has the ball and plays for the strikers, but obviously his [Costa's] impact was huge.
"It's a gamble to play him in a second consecutive game without at least a week of recuperation."
It will now be a race against time to see if Costa can feature in Tuesday's Champions League match with Sporting Lisbon.