“Doesn’t look good,” Coach Ben Olsen said in a text message.
The injury, witnesses said, occurred when an Orlando player challenged Arriola for the ball in the penalty area in the 30th minute of United’s 1-0 victory at the University of South Florida.
Arriola was treated on the field and then assisted to the bench, where he watched the rest of United’s fourth preseason game, a team spokesman said.
Olsen said Arriola probably will return home for the scans and long-term prognosis.
General Manager Dave Kasper, who is not with the team in Florida this weekend, said he did not yet know much about the extent of the injury.
The incident happened two weeks before the opener against the Colorado Rapids at Audi Field.
Arriola — a tireless, hard-charging player who joined the team from Mexican club Tijuana during the 2017 MLS season — contributed 13 goals and 10 assists over the past two campaigns. He has filled a number of starting roles: both wings, right back and central midfield.
The acquisition of Edison Flores, Julian Gressel and Yamil Asad, coupled with Arriola’s return, furnished United with one of the top attacking midfields in the league.
Arriola, 25, has also become a regular with the U.S. national team over the past two years and figured to play a key role in 2022 World Cup qualifying, which will begin around Sept. 1.
He recently rejoined United after spending a month on national team duty in Florida and California.
Whether short term or long term, Arriola’s absence will dent the D.C. midfield, which Saturday was without Asad and Flores because of injuries.
Asad might return for the opener; Flores, a Peruvian national team player and United’s big winter signing on a $5 million transfer from Mexico’s Morelia, should return next week for the last two tuneups: vs. Nashville on Wednesday and Philadelphia on Saturday, both in Clearwater, Fla.
The injuries probably will elevate winger Emmanuel Boateng and Ulises Segura on the depth chart. Acquired last summer from the Los Angeles Galaxy, Boateng began training camp behind several players. Segura, a Costa Rican national team player, has filled multiple roles the past two years.
Arriola’s injury overshadowed a 1-0 victory, United’s second consecutive triumph after two defeats. Guest forward Michael Gamble, from second-division Loudoun United, scored in the 77th minute.
Gamble, a former McDonogh School and Wake Forest standout who had entered two minutes earlier, also scored the lone goal against second-division Louisville on Feb. 6.