Webb is determined not to let that happen. He understands some of the criticism levied against V.A.R. during the Confederations Cup, but he is bullish on its future in M.L.S. if for no other reason than the officials’ readiness. They have reviewed hundreds of clips of contentious plays, participated in online webinars and practiced conveying their findings to the on-field official in concise language.

At minimum, each official will have refereed a game with a V.A.R., served as a V.A.R. during the M.L.S. preseason or in a United Soccer League match, and simulated three games as a V.A.R. with a surrogate referee. In all, Webb estimated, each official will have spent at least 100 hours training.

Several officials said the primary adjustment to the new role had been recalibrating their mentality. For so long they have been accustomed to making every call, but now they need to worry about only select situations.

“Your instinct at first was to re-ref the game, but that’s not what you’re there for,” said Geoff Gamble, an M.L.S. official. “You’re not trying to overturn every microscopic thing. It’s only the big, big things.”

Across two days of practical training and classroom sessions last week, Webb emphasized several points. He reiterated that V.A.R. is not intended to achieve 100 percent accuracy but to instead minimize mistakes like Carlos Tevez’s offside goal for Argentina against Mexico in the 2010 World Cup.

With a GoPro camera monitoring officials’ behavior and body language in the booth, Webb directed them to always project confidence when announcing a decision.