The 24-year-old Germany international moved to Stamford Bridge from Werder Bremen last summer, but the capture of compatriot Andre Schurrle from Bayer Leverkusen means he has fallen down the pecking order under Jose Mourinho.
Down the pecking order: Marko Marin has failed to make an impact since joining Chelsea last year
Marin made 16 appearances for the Blues last term after a hamstring injury early in the season delayed his competitive debut.
Schurrle this week became Mourinho’s first signing of his second spell as Blues boss, with more anticipated.
Willkommen: Chelsea have already splashed out this summer on Andre Schurrle from Bayer Leverkusen
With only youngster Jamal Blackman in place, Chelsea have been linked with Norwich’s England goalkeeper John Ruddy, who spent much of last season out injured with a thigh problem.