Diego Costa was fined £185,000 by Chelsea before he was allowed to rejoin the first-team squad, The Sun is reporting.
The Spain international trained on his own on Monday having been left out of the weekend trip to Leicester City.
His omission came following an alleged row with fitness coach Julio Tous over a back problem, with reports he had also had his head turned by a big-money offer from the Chinese Super League.
The Sun say Costa was fined a week’s wages as punishment before being allowed back into the fold once he had “ironed out his differences with [boss Antonio] Conte and Julio Tous”.
Chelsea have refused to comment on The Sun’s story.
After Saturday’s game at Leicester, which Premier League leaders Chelsea won 3-0, Conte insisted he did not know anything of supposed interest in Costa.
At his post-match press conference, he said: “In this situation you can listen to a lot of things.
“In my experience as a coach if there are the problems you solve them in the changing room not the press conference.
“Diego stopped his training on Tuesday because he felt a pain in his back and from that moment he didn’t train.”