An injury sustained in pre-season kept him out of the start of the campaign, and he has been left on the sidelines ever since.
He has made just one appearance, in the third round of the Capital One Cup against Wolves, where he played 22 minutes as a substitute in a 6-0 win.
It was expected that game would pre-empt his integration into the starting line-up, but that was on September 25, and over a month has passed without him making it onto the pitch.
He has been on the bench, but Di Matteo has simply chosen to go with other options as is his prerogative as manager. One game where we did expect to see him was the latest round of the League Cup last week against Manchester United.
It seemed the perfect chance for Marin to show what he could do in such a high profile encounter. Even with the game wearing down, and United's young defence exposed, the manager decided to go with Oscar and Hazard off the bench, rather than Marin.
It was a decision which paid off on the night, but perhaps the pair would have been fresher for last Saturday's draw at Swansea had Di Matteo gone with Marin.
With November now upon us, the countdown to the transfer window is soon to begin. Marin has two months to make an impact at Chelsea otherwise he could find himself loaned out in January.
There will be no shortage of takers, a middle of the road Premier League side like Aston Villa for example would love to bring him on board, and the experience of being the main man and gaining regular playing time in England's top division may be a good thing for the German.
Chelsea should really hope it doesn't come to that, and that Marin can stay fit and make an impact between now and then. He has the talent, and he hasn't even begun to show it, he just needs an opportunity.
Di Matteo must hand it to him sooner or later, even if it is a case of throwing him in at the deep end. Judging solely on his time in Germany, and his pre-season with Chelsea, he can be a star, and can be Chelsea's secret weapon.
It may make sense in late January to loan him if he is still not getting games. A tough run of fixtures means it will not be easy for Di Matteo to ease him in over the coming weeks. It could be sink or swim for the German.
How should Chelsea handle Marin given his slow start? Would a loan move be practical?
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