The Spain striker was dropped by Conte for last weekend's win over Leicester after an incident in training.
The club and Conte insisted Costa had a minor back injury but the 28-year-old is understood to have received a big-money offer to play in the Chinese Super League.
Chelsea fans react to Diego Costa comeback
Sun, January 22, 2017
Click through the gallery for the best reactions as Diego Costa returns for Chelsea
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Click through the gallery for the best reactions to Diego Costa's Chelsea comeback
But the forward was allowed to rejoin the main squad in training and made his return to starting XI against Hull yesterday.
And Costa broke the deadlock in first-half stoppage time with his 15th league goal of the campaign.
Thibaut Courtois is happy to have Diego Costa back
Courtois kept his 13th clean sheet of the season in the 2-0 win and says the speculation surrounding Costa never disrupted the team.
"We were very calm about it because we knew what was going on with Diego," said Courtois.
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Diego Costa scored his 15th goal of the season
"Speculation in the press is for the outside and maybe other teams find that stuff pleasing when it comes out.
"But we knew it was just a minor injury and sometimes you have to sit out one game. Yesterday he was back, fit and scored a goal so it was good.
"There was too much talk last week for him. He was just calm, working hard in training and he showed that in the game."