United will also ship allocation money to the Los Angeles Galaxy, which owns Arriola’s MLS rights because he was once a member of its youth academy. On top of that, United will furnish him a contract of more than $1 million annually through 2020, according to sources familiar with the pact.

United officials declined comment, but on his Twitter page late Wednesday, Arriola confirmed the move.

“I believe it is the right time to move on,” he wrote. “I am extremely excited for the new opportunity and the challenges that await me at D.C. United.”

A formal announcement is expected Thursday. United will open training to the public, starting at 10 a.m. inside RFK Stadium.

Arriola, a starter for Bruce Arena’s national team at the CONCACAF Gold Cup this summer, arrived in Washington on Wednesday to take a physical and complete the deal. After those two sides did their part, Tijuana submitted the final paperwork before a midnight cut-off.

Arriola will be classified as a young designated player, meaning he’ll count less than half as much against the salary cap as a high-end player age 24 or older.

The Arriola revelation came amid a flurry of activity by United (5-14-4) ahead of the league’s summer trade and transfer deadline. (Teams had until midnight to complete deals but did not have to announce them by that time.)

Late last week, United acquired Bolivian forward Bruno Miranda, 19, on loan from Universidad de Chile. This week began with the club trading attacker Lamar Neagle, last year’s leading scorer, to the Seattle Sounders for a 2018 fourth-round draft pick and longtime defender Bobby Boswell, the former captain, to Atlanta United for a 2019 third-rounder.

In both cases, United was looking to clear roster and salary cap space and not aiming for much in return.

Earlier Wednesday, United announced the signing of Hungarian national team midfielder Zoltan Stieber, 28, from Kaiserslautern, a German second-division club. He and Arriola will become United’s highest-paid players.

D.C. also acquired American defensive midfielder Russell Canouse, 22, from German club Hoffenheim on a free transfer.

While United has little hope of reviving playoff aspirations this season, the flurry of moves rocked the league and brought hope to a fan base that had grown disillusioned by the team’s lack of roster changes over the past year and frugal spending across several seasons.

United took the initiative now for two reasons: The team is in desperate need of help (winless in seven straight, fewest goals in the league) and, with Audi Field, a 20,000-capacity stadium, scheduled to open next summer in Southwest Washington, investors wanted to begin building momentum for the 2018 season. The new venue will provide greater revenue streams than available at antiquated RFK Stadium, allowing the team to make larger investments in players. The club has already sold the stadium naming rights and almost all of the 29 available private suites.

United acquired Stieber without having to pay a transfer fee. Initially, the club was prepared to pay more than $1 million for his rights. It’s unclear why the terms of the deal changed. For Stieber’s salary, D.C. will use targeted allocation money, which the league provides to teams to help offset the financial burden of a player earning between $500,000 and $1 million.

He arrived in Washington over the weekend and has been training with the squad. Both he and Miranda must wait for their work visas and international transfer certificates to be approved before becoming eligible to play. Needing only their ITCs, Canouse and Arriola should be available Saturday against Real Salt Lake at RFK.

Stieber is a left wing who is also comfortable on the right side and in the middle. His contract runs through the 2019 season, with a team-held option in 2020.

“Quality,” Coach Ben Olsen said, citing Stieber’s “ability to connect passes. He’s got a final product, and the game is pretty easy for him.”

The challenge, Olsen said, is integrating him into the squad and raising his fitness level. He has not played a full 90 minutes since April 16 against Union Berlin. His only action since mid-May came in a friendly against Russia and a World Cup qualifier against Andorra, both in early June.

Arriola, a 5-foot-6 flank player from Chula Vista, Calif., has played for Tijuana since spurning an offer from the Galaxy in 2013. On the international scene, he has served with the U.S. under-17, U-18, U-20 and U-23 squads.

Arriola made his senior national team debut in May 2016 and scored in a 2018 World Cup qualifier against Trinidad and Tobago last September. This year, after starting in World Cup qualifiers at Panama and Mexico, he was in the lineup for four Gold Cup matches, including all three games in the knockout stage. Arena is expected to summon him for critical qualifiers in early September against Costa Rica and Honduras.

United projects him as a left or right wing.

At age 16, Canouse left his Lancaster, Pa., area high school to join Hoffenheim’s youth setup in 2011. He played for the club’s junior team in the fourth division for three seasons and was called into the first team for one Bundesliga appearance in March 2016. Last season, he was on loan to second-division Bochum, appearing in 20 league matches (eight starts, one goal).

His arrival addresses one of United’s greatest weaknesses: deep-lying central midfield. Canouse began training with United on Wednesday.

Canouse and Travis Worra, United’s second-choice goalkeeper, were teammates at Hempfield High in Landisville, Pa.

Meanwhile, United and starting goalkeeper Bill Hamid have resumed contract negotiations after months of on-and-off talks. His current deal, worth $350,000 in base salary, expires after this season.

Hamid, a homegrown player from Annandale, has had a longtime interest in playing overseas, and with his U.S. national team assignment at the Gold Cup this summer, his profile has risen.

However, United would like to retain him as one of the club’s centerpieces at Audi Field. If Hamid, 26, signs abroad, United would retain his MLS rights. He is not eligible for league free agency, which requires both eight years of service and 28 years in age.