L'Equipe says Chelsea have made a €30 million (£26 million) offer for Tati ahead of tomorrow's transfer deadline day.
However, the offer was immediately rejected by Nantes.
With the rejection in place, a deal is now considered difficult to complete, not least considering the time aspect.
Chelsea had a plan to sign Tati and then loan him back to Nantes for the rest of the season.
But Nantes have other ideas for Tati and are insisting €30m isn't enough for their 18-year-old first-choice.