The Union got two goals less than 1 minute apart from Danny Cruz in the second half, erasing a 2-1 deficit.
But Rosales earned a point for the Sounders (1-3-3) with his first of the season, which came after he received a cross from Leonardo Gonzalez.
Eddie Johnson also scored for Seattle, which has only five goals so far this season.
Cruz's tallies were the first of the season by a midfielder for the Union (3-3-3).
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