WASHINGTON -- D.C. United tapped into its Argentine pipeline again Tuesday, signing midfielder Matias Donnet. The 26-year-old Donnet spent the last three years with Boca Juniors in Argentina and played one season with Venezia in Italy's Seria A. He has trained with D.C. United since Aug. 15 and will be available for Sunday's MLS game against Chivas USA. Donnet becomes the third player on United's roster from Argentina, joining midfielder Christian Gomez and defender Facundo Erpen. Lucio Filomeno was on the team at the start of the season, but he was released in June.