De Rosario, a former Toronto FC member, put United in front early, scoring less than a minute into the match when he headed a Branko Boskovic free kick into the bottom-left corner.
De Rosario made it 2-0 before halftime, collecting a pass from Chris Korb and scoring from 7 yards out for his fifth goal this season.
Andy Najar later set up Hamdi Salihi to seal the win for United (7-4-3), which has compiled a seven-match undefeated streak.
Danny Koevermans scored for Toronto FC (0-9-0), which ran its MLS record for losses to begin a season to nine games.
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