Costa's future at Stamford Bridge is in doubt following a bust-up with Antonio Conte and interest from China.
The striker returned to the side yesterday as Chelsea beat Hull 2-0 to extend their lead in the title race.
And international team-mate Pedro says the 28-year-old is happy at Chelsea and is showing the goods on the training ground.
Diego Costa is back in the Chelsea team and back among the goals
Diego Costa now has 15 Premier League goals for the season
“With the team, he has been training very well," Pedro said.
"He is very happy to be here in the club, this is the most important thing.
Chelsea fans react to Diego Costa comeback
Sun, January 22, 2017
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“His focus in the team, the game, this season to fight for this title.
"It’s good for the team. This ambition Diego shows is good for the team.”