According to the Daily Telegraph's Matt Law, Chelsea could sue Costa "for effectively wiping millions of pounds off his valuation."
The Blues could consequently seek "the difference between his sale price and market value if it was felt he eventually left at a reduced price" as a result of his actions this summer, though they "are not currently looking at taking any legal action," Law noted.
However, per Richard Parry of the Evening Standard, Costa stated via the Federacao Sergipana de Futebol: "My destination is already made. I must return to Atletico Madrid. It turns out that there is the impasse that Chelsea does not want to release me. But I believe that this situation will be resolved now on my return to Spain."
Sky Sports News' Kaveh Solhekol reported Chelsea have already issued a fine to the striker and made their demands of him clear:
Kaveh Solhekol @SkyKaveh
Diego Costa has been fined by Chelsea for not returning for preseason training last month. Likely to be two weeks wages - so £300,0002017-8-14 14:05:10
Kaveh Solhekol @SkyKaveh
Chelsea want Diego Costa to come back to UK, start training, get match fit and put himself back in contention for selection2017-8-14 17:25:43
Despite being unable to play for Atletico until January because of their FIFA-imposed ban on registering new players, Costa has his heart set on returning to the Spanish capital.
According to Spanish outlet La Voz de Galicia (h/t Fox Sports), Deportivo La Coruna have opened talks regarding a loan deal until January. Law reported Real Betis are also interested in signing the player on a temporary basis.
Such is Costa's desire to rejoin Atleti, he may not be willing to consider any move in the meantime.
In terms of the 28-year-old's valuation, Chelsea should be able to fetch a hefty sum for him on the back of his performances last season. He made a vital contribution to the Blues' Premier League title win, as ESPN FC's Liam Twomey noted:
Liam Twomey @liam_twomey
Diego Costa's 20 goals directly led to 16 extra points for Chelsea in the Premier League last season. They won the title by seven #cfc2017-8-16 10:40:28
However, by revealing the Blues' hopes of moving him on and his consequent refusal to report for training, Costa could have significantly reduced the amount suitors are prepared to pay for him.
It appears the Spain international is willing to force his way out of Stamford Bridge. But working with Chelsea rather than against them could lead to a smoother departure from the club.