The 17-year-old is said to be the latest jewel to emerge from Brazilian football and is widely tipped to have a big future ahead of him at the top level.
Reinier only debuted for Flamengo - who are in this year’s Copa Libertadores final and top the Brazilian top-flight - earlier this year, but he already has three goals in just four starts in the league.
Reinier has a termination clause of 70 million dollars (approximately €62m) at the Brazilian giants but Atleti are said to be confident in being able to reduce this drastically in a theoretical transfer.