Webb, a former Premier League and World Cup referee, blamed video assistant referee Geoff Gamble for not alerting head referee Armando Villarreal to a possible infraction.

“There is no doubt the VAR should have recommended a review,” Webb said. “That would have been the trigger point for the referee to look at it and make his own final determination. I’m pretty confident if that had happened, the outcome would have been different.”

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The mistake came about a month after the referees’ organization admitted mistakes with video replay in consecutive United matches: A D.C. goal during a 1-0 defeat at Minnesota should have counted, it said, as well as a Columbus score during a 3-1 loss in Washington.

On the pivotal play Wednesday, Calvo was in the process of stepping back into an onside position when teammate Bastian Schweinsteiger flicked the ball toward him. Calvo beat goalkeeper Bill Hamid to the ball and headed it into the net, ruining first-place United’s three-goal rally at Audi Field and extending D.C.'s winless streak to four.

“He doesn’t quite get back onside in time,” Webb said. “It’s not by a huge amount, but it is enough [to be ruled offside], in my opinion.”

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United did not defend the play well. Had defender Jalen Robinson stepped forward, there would not have been any doubt Calvo was offside. Also, Hamid was slow in charging for the ball.

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It was also an unusual sequence in that Calvo was trying to get back onside. Typically, a player begins onside and is trying to avoid being offside.

“To the naked eye,” Webb said, “it didn’t look offside.”

The assistant referee kept his flag down. It was close enough, however, that Gamble, with access to multiple replays behind the scenes, should have recommended a review by Villarreal. Instead, without the alert from Gamble, Villarreal allowed the goal to stand and restarted play almost right away.

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“VAR has done an amazing job in many, many situations to rectify clear errors,” Webb said, “but this is an occasion where the human being who is the VAR [official] didn’t recognize the error.”

MLS implemented VAR late in the 2017 season. The Professional Referee Organization is an independent body that oversees officiating in the United States and Canada.

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PRO does not use offside technology, Webb said, because “the software available to us but not reliable enough. We are looking into it in some recently developed software.”

So the video referee must rely on his instincts and observations to flag possible infractions and, when applicable, alert the on-field referee.

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Earlier on Wednesday, video replay nullified an apparent D.C. goal. Lucas Rodriguez appeared to score a clean goal just before halftime, but Gamble noticed the midfielder’s arm had made contact with the ball an instant before he knocked it into the net. Villarreal watched replays for himself and waved off the goal.

PRO evaluates referees on several criteria, including video-replay decisions.

“It’s a performance industry,” Webb said, “and they know their continued employment is performance related.”

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In the big picture, Webb said, VAR has corrected more wrongs than it has missed. But it remains a work in progress.