The German midfielder arrived at Stamford Bridge in a £7m deal from Werder Bremen back in 2012 but he has never established himself at the club.
He has since spent time out on loan at Sevilla, Fiorentina, Anderlecht and Trabzonspor with varying levels of success, while appearing for the Blues just six times in the Premier League.
And now the 27-year-old has left Chelsea for good after signing a three-year deal at Olympiacos, who agreed a cut-price fee for the Germany international.