Atlético Madrid and Diego Costa have some history together, most notably a La Liga title and a run to the Champions League final. Immediately following that historic season, Costa moved to the Premier League, as his play over the previous two season had raised his expected wage to a level which Atletico just couldn't match.
While Costa has performed admirably on the pitch during his time in England, his demeanor and behavior have become controversial enough that he has reportedly become unsettled. The Spanish media are far more accepting of Costa's antics, however, and we've seen quite a bit of chatter about an Atletico return, as a result.
While an Ateltico return might be something that the Spanish side desires, it's probably not a deal that the Madrid side could actually complete. Club president Enrique Cerezo confirmed as much on Tuesday:
"He has a contract with Chelsea. There are many times when, as much as a I want a player, there is a contract. To negotiate with Chelsea can be really tough, almost impossible."
-Enrique Cerezo; Source: Onda Cero via FourFourTwo
Reports claiming that Costa wants out of Chelsea and England will continue to be published for as long as the player remains in England, as the media just can't stop writing about what a horrible guy Costa is. But given how he's capable of playing, the resources that Chelsea have, and the lack of high quality replacements available, a move is highly unlikely, unless someone comes along with an offer so ridiculous that the Blues just can't say no. That offer isn't likely to come from Costa's former club.