"Everton are the latest Premiership club to express an interest," Mills told the Nottingham Evening Post.
"But they are certainly not the only ones. There has also been contact with Fulham and Spurs.
"He is a player a lot of clubs are keen on but we have not had any offers yet.
"It is impossible to say which way things will go. Perhaps it will end up going to a tribunal - but if we receive an acceptable offer in the meantime, we will obviously consider it."