Maximiliano Pereira, 23, from Defensor Sporting of Montevideo, signed a five-year contract worth $4.10 million. Cristian Rodriguez, 21, signed a one-year contract in a free transfer from Paris St Germain.
Benfica did sell midfielder Manuel Fernandes to Valencia for a reported $25 million.
Adu was reunited with Coach John Ellinger, Adu's U-17 national team coach, when he joined Real Salt Lake for the 2007 season after three years with D.C. United. But Ellinger was replaced by Jason Kreis after five games.
Adu joined Benfica on $2 million transfer. So far he has seen action in a Champions League qualifying game.