The 23-year-old played for the Bundesliga outfit last season but his spell was cut short after he suffered a serious knee injury in January while on international duty with Ghana in the Africa Cup of Nations.
Schalke sporting director Christian Heidel has claimed that the club are speaking with the Blues about a potential deal for the defender, but admits that nothing has been agreed.
Heidel is quoted by the Daily Mail as saying: "We are in talks with the player, his agent and Chelsea. Ultimately, it is about when Baba will be able to play again. No decision has been made yet."
Rahman's current contract with the Premier League champions runs out in 2020.