The goal from Santos came 4 minutes after Bernardo Corradi had put Montreal in front on a header, but the Impact came up short in its bid for the second win in franchise history.
United extended its season-opening unbeaten streak to five games (2-0-3) and moved within a point of New York for second place in the Eastern Conference.
Montreal fell to 1-5-2 in its inaugural MLS season.
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