Chelsea have reportedly been considering a £50m lawsuit against Diego Costa if the striker refuses to return to Stamford Bridge.
The 28-year-old has stayed away from the club after a fallout with manager Antonio Conte over the player's Blues future, reportedly conveyed to him by text.
A prospective move to former employers Atletico Madrid was said to be on the cards, but no deal was secured before the closure of the transfer window, keeping Costa on Chelsea's books.
According to The Express, Chelsea would claim breach of a contract from which Costa is paid £185,000 a week - the club's second highest earner - and which still has two years to run.
The Spain international, who has been named in Chelsea's 25-man Premier League squad but not in their Champions League one, is now also understood to be consulting his lawyers.
Costa has scored 52 goals in just 89 Premier League appearances for Chelsea, including 20 to help fire them to the title last season.