MacMath got a concussion when struck with a knee in the head during a May 8 game against Seattle. He did not travel with the team to Texas.
The 20-year old participated in last week's game against the New York Red Bulls even after being labeled "out" during an injury report published early in the week. But ESPN color analyst Taylor Twellman reported this week that MacMath took a hit during practice to suffer his setback.
With MacMath out, the Union will turn to Chase Harrison or Chris Konopka. Neither has played a single minute this season, as MacMath has not come off the field for the Union this year.
FC Dallas will also be without its goalie, Kevin Hartman.
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