The 25-year-old was linked with Crystal Palace earlier this year, while Arsenal are also said to be scouting him.
The Frenchman already has four goals to his name this season and he has impressed many with his performances.
Indeed, according to Calcio News 24, Leicester, Liverpool and Napoli are now all keeping tabs on Defrel.
Sassuolo are well aware of the growing interest in their star striker and they will not let him go on the cheap.
The Italian club are said to want around £16m for Defrel and will not entertain any bids which do not match that amount.