United’s Ian Harkes collides with Houston’s Alex at RFK Stadium on July 22. (John McDonnell / The Washington Post)

D.C. United midfielder Ian Harkes, who has started all but three matches in his rookie season, will miss three to four weeks with an ankle injury, Coach Ben Olsen said Tuesday.

Harkes, a homegrown signing from Gonzaga High School and Wake Forest University, was injured early in the 4-0 defeat at Minnesota on Saturday and did not return for the second half. He has posted two goals and an assist in the past seven matches.

Meantime, midfielder Rob Vincent is out indefinitely with recurring knee problems. He hasn’t played since the opening month of the season and, after undergoing routine knee surgery in late March, developed a blood clot that sidelined him several months. The Englishman returned to active duty last month before suffering a setback in his left knee.

“He just has a bum knee,” Olsen said. “It’s bone on bone. Every time we get him up to a certain speed, it swells up.”

>> Midfielder Patrick Nyarko, one of the few bright spots in the attack, remains out after being elbowed in the eye during a match against Houston on July 22, the team said. He suffered a scratched cornea, but because of a history of concussions, the team is taking a cautious approach before clearing him to play again, Olsen said.

Defender Taylor Kemp, the starting left back, was given the day off Tuesday to rest a hip injury. He is expected back this week.

>> Veteran defender Bobby Boswell, on United’s mood after six consecutive defeats, 15 goals conceded in four matches and fading playoff hopes:

“It’s not good. Everyone understands that. You’ve got to tighten up the back, stop leaking goals and you’ve to get results. Getting blown out and giving up all of these goals, it’s a killer for the team. If we can get everyone on the same page, I’d like to think we can get something out of this year. It’s got to start with belief and optimism. I hope we’re at the rock bottom here and go up from here.”

>> Near the end of Tuesday’s training session — United’s first since the lopsided defeat at Minnesota — frustration boiled over as captain Steve Birnbaum began yelling at Argentine playmaker Luciano Acosta. The incident lasted less than a minute. As it died down, Acosta walked to the bench area to retrieve his gear and promptly left the facility.

At first, it appeared Acosta had departed without permission. Afterward, Olsen said, by that time, the regulars had been excused.

“A lot of chatter today,” he said, “because guys are frustrated.”

>> For the second time in a month, United arrived for an away match on the same day as the game because the scheduled commercial flight the day before had been cancelled. There were no later flights available to Minneapolis, so the club scrambled to arrange a charter Saturday morning.

A similar problem played out ahead of a trip to Montreal for a July 1 game.

An antiquated league rule limits teams to four charters per season. The purpose is to foster competitive balance, but the league is all grown up now and every ownership group is stacked with enough cash to cover the cost of upgraded travel.

“This league has to do charters. This is a joke,” Olsen said. “They really need to figure this out. Strike a deal with a charter company, give them some sponsorship. Figure it out. Every game should be chartered. It’s a joke. It’s too important now. You’ve got players who are used to charters [playing overseas and with national teams]. You got guys making seven million dollars waiting around around for commercial flights to take off.”

>> The MLS all-stars will face Real Madrid on Wednesday at Chicago’s Soldier Field (8:30 p.m. ET, FS1). None of the 24 league selections came from Washington. United didn’t have any representatives in Tuesday’s 2-2 draw between a homegrown squad and Chivas Guadalajara’s under-20s either. Harkes, 22, would’ve been a prime candidate, but coaches Brian McBride and Mike Magee wanted to showcase younger players.

>> RFK Stadium’s playing surface will have to endure one college football game this fall: Georgetown vs. Harvard on Sept. 30. United does not have any home matches scheduled for a four-week stretch leading to the Oct. 22 season finale against the New York Red Bulls.