“It’s disappointing,” Olsen said. “I am disappointed for him. He was in a good rhythm and each game was getting a little bit better. It was nice to see him grow.”

On the recovery of the 6-foot-5 center back, Olsen expressed optimism — “He is young” — and then added with a grin, “It’s probably a big ligament or a long ligament.”

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Pines apparently was injured during the 3-1 victory over the Columbus Crew on Saturday at Audi Field. He played all 90 minutes and seemed fine afterward. However, discomfort led to the discovery of the ligament issue.

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He is the fourth defender to suffer an injury since the start of the season. First-choice left back Joseph Mora (broken jaw) will not return until late June at the earliest; backup left back Chris McCann (hamstring) just returned to game-day duty; and right back Leonardo Jara missed one match with a strained hamstring and one half of another with a groin injury.

Also, right backs Oniel Fisher (ACL recovery) and Chris Odoi-Atsem (cancer treatment) have yet to play this season.

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With Pines sidelined, Olsen is expected to return to a four-defender formation after playing with three center backs and two wings for most of the past month. This Sunday against Sporting Kansas City, he will probably pair Steve Birnbaum and Frederic Brillant in the middle, use Jara on the right corner and perhaps newly acquired Marquinhos Pedroso on the left.

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“We adapted to our outside backs getting hurt, and now a center back gets hurt,” Birnbaum said. “It’s a tough blow for us, and you feel bad for him because he was doing well. His confidence has been growing.”

>> With MLS’s first transfer and trade deadline late Tuesday night, United General Manager Dave Kasper was working on a potential acquisition from overseas, multiple people close to the team said.

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No details were revealed. The club has remained interested in midfielder Yamil Asad, who started for United most of last season while on loan from Argentine club Velez Sarsfield. One person close to the team said Asad was in the mix early in the spring, faded a bit from consideration, then reemerged.

United’s primary pursuit, however, was not Asad, that source said.

Some deals around MLS that are consummated at the 11th hour are not announced until the next day.

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Teams will have another opportunity to bolster the roster when the month-long summer window opens in early July.

Asked whether United (6-3-2, tied for first place in the Eastern Conference) faced any urgency in adding players now, Olsen said: “Absolutely. It’s an urgency you always have to have now in this league. That’s just the way the league is going. We’ve been able to manage the season so far and grab points, but at times we’ve gotten them the hard way.”

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>> Griffin Yow, United’s 16-year-old midfielder who signed a contract in preseason, scored twice for the U.S. under-17 national team in a 3-0 victory over Guatemala at the U-17 World Cup qualifying tournament in Bradenton, Fla.

The Americans (3-0-0) will face Guadeloupe on Thursday in the round of 16.

Midfielder Bryang Kayo, a United academy pupil, played 79 minutes.

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Midfielder Chris Durkin is expected to report to the U.S. under-20 national team next week for the buildup to the U-20 World Cup in Poland, from May 23 to June 15. The U.S. Soccer Federation has not announced the squad, but Durkin is almost certain to be a selection.

If chosen, United officials said, he would be available for matches Sunday against Kansas City and next Wednesday at Toronto.

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>> Fox Sports 1 will carry Sunday’s match. Coverage will begin at 7 p.m., but kickoff is not until 7:25. An all-women crew will call the game: Jenn Hildreth, Cat Whitehill and Jillian Sakovits.