Bernardo Corradi had given the Impact the lead in the 68th minute when he took advantage of Chris Korb's ball-watching and Joe Willis' over-aggressiveness to head Lamar Neagle's cross from the right wing into an open net.
Santos had come on as a 60th-minute substitute in place of Branko Boskovic, and he leveled the score in the 72nd minute. Santos tried a left-footed shot from outside the penalty area that skipped past the futile dive of Donovan Ricketts and into the corner of the net.
Santos thought that he had given United the win in the final minute of normal time, but he was correctly ruled offside as he buried Dwayne De Rosario's cross from the right wing.
United has not won two consecutive MLS games since 2009.