The 21-year-old is thought to have several suitors throughout Europe after a series of impressive performances in helping Cruzeiro to back-to-back Brazilian Serie A titles.
Premier League clubs Manchester United and Arsenal have both been linked with a swoop, but European champions Real Madrid are thought to be leading the race for the midfielder.
Tavares insists the youngster, who has represented Brazil at under-20 and under-23 level, will not be sold on the cheap, though, and said his price tag had been made clear to all potential buyers.
"[There has been] no final proposal," he is quoted as saying by Spanish newspaper AS.
"We already put [his] price on the market. For less than €15 million, Cruzeiro will not begin to talk."
Real have been in stunning form this season, with Carlo Ancelotti having guided them to a club record 17 wins in a row in all competitions.
They sit top of La Liga with 33 points from 13 matches, during which they have scored 48 goals.
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