Diego Costa was named to Chelsea's official Premier League squad list despite the Spain international's attempts to force a move to Atletico Madrid this summer.
Chelsea reportedly fined Costa two weeks' wages for failing to return to training, and has remained in his native Brazil as the dispute over his future rumbles on.
Costa claimed in an interview with with the Daily Mail last month that Chelsea are treating him "like a criminal" and that he has lost all personal respect for head coach Antonio Conte over the Italian's handling of the situation.
Sources at Chelsea told ESPN FC last month that four conditions have been set out for Costa to resolve the current stand-off.
The first is that he return to England, the second that he return to training, the third that he get himself match fit and the fourth that he make himself available for selection.
However, with Spain's transfer window closed, the 28-year-old striker has been officially registered by Chelsea for the season.
Upon seeing Costa's addition to the team, Chelsea goalkeeper Thibaut Courtois poked fun of Barcelona defender Gerard Pique's now-infamous "se queda" Tweet.
The former Atletico Madrid man took to social media and posted an image of him and Costa with the same caption.