Arriola, 25, underwent an MRI exam Sunday morning in Florida, a team spokesman said. The extent of the damage and long-term prognosis were not immediately disclosed.
However, a person close to the situation said Arriola has a partially torn ACL and no other damage, a diagnosis that could allow him to return sometime this summer.
“I had some preliminary testings, and I’m waiting to speak with some doctors and other physicians to see how we should move forward with the injury,” Arriola said.
No one else associated with the organization wanted to comment until Arriola and team officials consulted with medical specialists on the course of action.
Even under the best circumstances, Arriola would need quite some time to recover and regain his fitness and form.
His value comes not in statistical production, though he has steadily refined his skills in the final third of the field and scored 13 goals in the past two seasons combined.
Arriola parlays an abundance of speed, energy and determination into dashing runs on the wings. He also combines with teammates, most notably in 2018 and 2019 with Wayne Rooney and Luciano Acosta, to create opportunities.
With an engine that never stops humming, he takes defensive responsibilities as seriously as attacking tasks and tracks back to thwart opposing forays. In other words, he is all over the field.
Late last season, with playmaker Acosta falling out of Olsen’s favor, Arriola embraced a role in central midfield. In 2½ seasons with United, he also has filled needs at right and left back.
U.S. national team coach Gregg Berhalter also utilized Arriola’s skill set and versatility.
During the Gold Cup last summer, Berhalter started Arriola on the left wing in all but one match. Two weeks ago, he started him on the right side in a friendly against Costa Rica.
Though Arriola is not essential to the U.S. squad, which draws players from both MLS and European leagues, the injury is a possible blow to Berhalter’s broad plans steaming toward the start of 2022 World Cup qualifiers late this summer.
Arriola will miss friendlies late next month at the Netherlands and Wales and is all but certain to miss the Concacaf Nations League final four in June with Mexico, Costa Rica and Honduras.
The more immediate concern is United. Last week, Olsen spoke excitedly of the team assembling several versatile midfielders, including Arriola, in providing a variety of lineup options.
Among the winter acquisitions was Julian Gressel, who, like Arriola, can play multiple positions but is most comfortable on the right wing.
What to do?
“They will both be on the field,” Olsen said last Monday. “Versatility helps in scenarios like this. They have some similar qualities. But Paul does more of his damage without the ball, and you could say Julian does it with the ball.”
In all likelihood, Olsen was going to leave Gressel on the right and play Arriola on the left, especially with left wing Yamil Asad recovering from a groin injury.
(Arriola, however, would not have played in the Feb. 29 opener against the Colorado Rapids at Audi Field; a red card in the playoff defeat at Toronto FC last fall resulted in a one-game suspension this season.)
As gutting as Arriola’s injury has left United, all is not lost.
Asad is in his second tour with United after a standout 2018 season. Ulises Segura, a Costa Rican national team selection this winter, has played several positions the past two years.
Emmanuel Boateng, a speedy winger, has made 65 MLS starts since the 2016 season began. Griffin Yow, 17, is a homegrown prospect.
And there is Edison Flores, United’s $5 million acquisition this winter. United plans to play him in central midfield, though he is a left wing for the Peruvian national team.
Olsen said last week, “It’s finding the balance and putting these pieces together to start producing opportunities in a way we weren’t able to really master last year,” when United scored the fourth fewest goals in the league.
“We’re a work in progress,” he added. “Like most teams. Maybe like every team. Some coaches have it all figured out already. I am being candid: We’ve got a lot of work to do over the next few weeks.”
Without Arriola, that workload just became much heavier.