Portuguese midfielder Mario has been an expensive flop at Inter.
He was signed for €40 million (A$65m) in 2016 as one of the first big moves made by new owners the Suning Holdings Group.
The 27-year-old never established himself in the nerazzurri's starting XI and was sent on loan to West Ham in 2018. He is currently on loan at Lokomotiv Moscow in Russia.
He blasted Inter's treatment of him in an interview with Portuguese publication A Bola, saying: "I could not train or play in friendly matches like everyone else."
"I trained alone with Mauro Icardi. I don’t understand their position, because Inter paid €40m for me and treated me like a kid.
"It was despicable.
"I found a less stable club than Sporting CP; a lot of confusion, change of coaches, a badly prepared season.
"I’m sorry that much more happened off the pitch than on it."
