The 28-year-old has struggled to command a regular starting spot in two seasons at Stamford Bridge and Xabi Alonso has already brought in Maxence Lacroix to bolster his defence.
Valentin Barco, Pep Chavarria and Marco Palestra have also arrived over the summer and Tosin's chances under Alonso look limited for the 2026/27 season.
Chelsea now have nine senior central defenders in place and Alonso has publicly stated he wants to reduce that number to "four or five", with the club not involved in European competition.
Tosin still has two years left on his contract, so Chelsea will be looking for a sizeable transfer fee, with interest from an unnamed Premier League club and Portuguese giants Benfica - as per BBC Sport.
A switch to Lisbon would reunite Tosin with his old Fulham boss Marco Silva and they can offer him UEFA Europa League football if a £15M valuation is met.
Fellow defender Axel Disasi and Benoit Badiashile are not currently training with the first team squad, and are available for transfer, while Mamadou Sarr and Aaron Anselmino could leave on loan.