It was then that Arena relayed to his players two critical decisions. The first was that he would be definitely overhauling the lineup after the qualifier against Trinidad and Tobago, replacing at least seven players but as many as the full 11 for the game at Mexico three days later. The swift turnaround, the hectic travel, the altitude — it all conspired to demand action. He informed the players of his plans, and — perhaps getting the not-too-subtle hint there were jobs to be won — they got to work.

If he could not field a first-choice squad, Arena had reasoned, he would start as many players as possible who had experience at Azteca. Seven of the 11 starters on Sunday had played there, either for national teams or in club play, and five had appeared during the Americans’ last visit here, a 0-0 draw in 2013.

One of those newcomers, midfielder Paul Arriola, anticipated that his recent form with Club Tijuana and his familiarity with Azteca would improve his likelihood of playing. Summoned for this camp, Arriola said he knew Arena had a plan for him, even if he didn’t quite know what that plan was until one morning last week. During breakfast one day, Arena came to him — as he did with others, individually, Arriola said — and explained Arriola’s role.

“I knew that I would get a chance,” Arriola said. “It was huge. It was a huge boost of confidence.”

Arena’s other notable move was more or less dividing players into two teams — one that would play a more traditional 4-4-2 formation against Trinidad and Tobago, and one that would play with a five-man back line against Mexico. Arena raised this idea with his coaches in January, he said, and it was not well received.

“They maybe don’t have enough experience in that formation,” Arena said, listing the main objection. “I was pretty confident we could implement that.”

After analyzing Mexico’s style — and assessing his own team, which is deep and strong at center back, where Geoff Cameron excelled on Sunday — Arena concluded that by employing three sturdy center backs and two speedier wings outside them at the back, instead of the traditional back four, the United States could prevent Mexico from stretching out the Americans and attacking the resulting gaps.

The team spent the last two weeks in training, Bradley said, making sure all the players — the defenders, but also the midfielders and forwards who would help track back to provide cover — understood their responsibilities.