The 21-year-old signed for the Hoops last summer after Celtic paid a £500,000 development fee following the expiration of his Fulham contract.
Dembele has three years left on his current Celtic deal, while Chelsea and West Ham United tried to sign the forward in January.
French striker Dembele scored 32 times last season to help Celtic win a domestic treble.
In January 2016, Dembele came close to joining Tottenham but the club were not prepared to meet Fulham's condition that he be loaned back to the club for the remainder of the season.
But it is believed Celtic boss Brendan Rodgers would not be prepared to sell Dembele until after their final Champions League qualifying game in August.
