The Portugal international has been linked with a move to the Premier League, with Manchester United, Chelsea and Liverpool all reportedly interested.
The 23-year-old came through the youth system at Sporting and is set to represent his nation at Euro 2016.
His father and agent, Joao Mario Eduardo, said the Primeira Liga giants were mistreating his son.
"Joao is hurt by Sporting," he told A Bola.
"It is normal that he is hurt. He is insufficiently valued. Sporting say Joao is worth 60 million and pay him like 10."
Mario could yet remain in his homeland, but his father is unsure if that is what Sporting want.
"We are prepared for Joao to stay another year in Portugal, but Sporting have to recognise his importance and pay the fair value," he said.
"I don't know if Sporting are ready for Joao to stay another year."