Costa is back in the Chelsea fold after a week-long exile prompted by a spat with manager Antonio Conte.
The striker was offered a £30million deal to go to China and his agent was spotted meeting Tianjin Quanjian's owner.
The furore has died down, however, and Costa is in line to feature against Hull tomorrow (4.30pm).
Diego Costa snapped with Antonio Conte in training
Tue, January 17, 2017
Click through the gallery for the best snaps of Diego Costa's training ground reunion
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Click through the gallery for the best snaps of Diego Costa's training ground reunion
And ex-West Ham star Upson is surprised the Spain international, who has scored 14 league goals this season, would want to quit Chelsea.
“I find it quite remarkable being at a club like Chelsea and challenging for the Premier League title – there shouldn’t be a greater time in your career than experiencing that,” Upson told Radio 5Live.
“Teammates always understand people wanting to better themselves, whether it be from a professional point of view or a financial one.
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Chelsea striker Diego Costa is back in the first team
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Costa (left) was wanted by clubs in China
“Maybe you get towards the end of your career and it becomes the most important issue in your career.
“You can’t criticise him for wanting something different if that was the case.
“But he’s in such a strong position in a great Chelsea team that could go on and win the title.
“If that’s the worst thing that could happen – you win the title with Chelsea at the end of the season – then I’m sure everyone will be happy.”
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