Unfortunately we have to lead with the caveat that the sourcing is very, very dodgy on this. The Turkish media, in general, are some of the least reliable in Europe when it comes to transfer rumors. So even though at least three of them are running with this story -- as collected by the intrepid folk over at Sport Witness -- this remains a very speculative story.
That being said, it's a fairly exciting one.
The story goes that 22-year-old Hakan Calhanoglu has been identified by Chelsea as the replacement for Oscar, who appears to be on his way out of the club potentially. So that's actually twice the speculation for the price of one, as it's hardly a certainty that Oscar will be leaving, though that story has its origins in far more reliable sources than this Calhanoglu one.
As relayed by the Daily Star, the local media also claim that Chelsea were close to sealing a deal for Calhanoglu this summer, with the player supposedly even telling his national team coach Fatih Terim about it. But all that proved premature as Leverkusen rejected Chelsea's low-ball €20m offer. The young attacking midfielder is certainly worth a fair amount more than that!
The one thing the reports seem to all agree on is that the ambitious young man wants to play in the Premier League, so linking him with Chelsea seems quite logical. He would no doubt be an excellent signing.