Deco"s B sample has tested positive for furosemide, a powerful diuretic that is used to hide performance-enhancing substances.
The Fluminense star is set to be automatically suspended for 30 days when Rio de Janeiro"s Sporting Justice is notified on Monday.
The test came on March 30, following Fluminense"s 2-0 win against Boavista for the Rio de Janeiro State Championship.
His supension will be confirmed only on Monday, but the former Chelsea midfielder will miss the Taça Rio final on Sunday although he would be eligible for the game.
Fluminense"s executive director Rodrigo Caetano explained this decision, he said: “There is no chance [he plays on Sunday], this option has been ruled out. His mind won"t be there, we have to preserve both the player and the human being."
Deco has already hired a lawyer to defend him, but Fluminense promised to keep supporting him.
If found guilty, Deco could be suspended for up to two years, which may encourage him to retire from the game.
Source: Globo Esporte
