Niasse is said to have had run-ins with several of his teammates, causing a deep divide within the Toffees' dressing room.
The Sun reports that it is a "war zone" behind the scenes at Goodison Park, with young midfielder Tom Davies among those prepared to hand in a transfer request unless the matter is resolved.
Davies is rumoured to have told a close friend: "Had enough of all the shit now, it's making me feel down every day."
Other senior pros will join Davies is pushing through an exit, it is claimed, as the likes of Phil Jagielka, Dominic Calvert-Lewin, Jordan Pickford, Seamus Coleman, Mason Holgate, Michael Keane and Jonjoe Kenny are all unhappy.
Another senior player is quoted by the publication as saying: "I say everyone chucks transfer requests in unless Oumar goes. He's the problem. He goes, problem goes."