Blues boss Antonio Conte is said to be keeping tabs on the talented 27-year-old, but there is also interest from elsewhere, with Milan also said to be in the hunt.
Badelj has just 18 months remaining on his current deal and some reports have suggested that Fiorentina could be forced to cash in, rather than risk losing him for nothing.
Chelsea could launch a January bid, but according to the player's agent, it seems as though Milan will have first option should Fiorentina decide to cash in.
When quizzed about the player's future, Dejan Joksimovic said: "I honestly do not know. We talk about a lot of teams interested in him. But I promised that Milan do not talk with any club on January 1.
"We do not know if Fiorentina will it sell in January or not."
Badelj has made 15 appearances so far this season, scoring one goal and providing one assist, and his performances have caught the eye of Conte.