The club claim the coveted attacking midfielder has joined Luis Enrique's team, with reports also stating the youngster has swapped Buenos Aires for the Italian capital
The 19-year-old has been the subject of intense transfer speculation ever since his side were humiliatingly relegated from the top flight of Argentine football and it has now been confirmed that he has moved to one of Europe's elite clubs.
In quotes published by Corriere dello Sport, Passarella said: "Lamela is a Roma player. The contract just needs to be settled with the federation.''
Meanwhile, Italian television station Sky Sports 24 claims the youngster has signed a five-year contract at the Stadio Olimpico. It will reportedly earn the player €1.3 million (£1.1m) per season, binding him to the capital club until 2016.
The television channel also added that an official announcement by Roma or the league itself may not come until Monday. No details of the cost of the transfer have been released, either, but a fee in the region of €10m (£8.8m)on plus add-ons has been reported.
On Friday, Sky Sports 24 had revealed that the midfielder would put an end to all speculation regarding his future and finally sign for the Roman club the following day.
The teenager has played 36 times for River Plate, scoring five goals. He has also won an international cap for Argentina on his way to attracting attention from a host of top European sides.