The Napoli centre-back is out of contract in 2023 and his club are open to offers.
However, the Daily Express says both clubs are remain 'so fond' of the Senegal international.
Napoli's original asking price of £100m had long deterred suitors from signing Koulibaly.
But it is believed the 29-year-old would be now be available for €50-60m.
With Gennaro Gattuso likely leaving Napoli this summer, it is believed a number of star players will follow suit.
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