Gremio are said to be ready to report Barcelona to FIFA after their player, Arthur, was pictured in the latter's jersey.
As we can see here courtesy of AS, the 21-year-old was snapped in a Blaugrana shirt on Friday:
David F. Sanchidrian of AS reported that the image was put out by Globoesporte. The snap emerged after Barca's technical secretary, Robert Fernandez, was in Brazil to watch the player in person.
Speaking to Estudio Gaucha (h/t Sanchidrian), Gremio's legal adviser Nestor Hein has said the club will be taking action:
"This young lad Arthur is our player.