Medical staff came to his aid and escorted him off the field with what was later diagnosed as his second concussion of the season.

Birnbaum became the third United starter in the past four weeks to suffer a head injury and the latest in a long list of D.C. players over more than a dozen years to miss time — or contemplate retirement — with a head injury.

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The central defender will sit out Wednesday’s match against Atlanta United at RFK Stadium and won’t return to active duty until he passes league-mandated concussion protocol. Forward Deshorn Brown this week was cleared to resume normal activities after missing one game, but midfielder Patrick Nyarko is sidelined indefinitely with issues resulting from his seventh concussion since 2008.

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For the time being, Birnbaum is limited to light conditioning and running.

“The symptoms went away afterward,” he said Tuesday. “I felt fine the day after and I feel fine today. I’ve been normal since maybe an hour after the game ended.”

That said, he is following the team’s recovery regimen and isn’t sure when he will return to active duty — perhaps this weekend against New England but more likely after United’s two-week break.

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Given its history and the heightened awareness of head injuries, the club isn’t taking any chances.

“When we talk about the head, we are extra precautious whenever the ‘concussion’ word is mentioned with any injury,” Coach Ben Olsen said. “When we’re dealing with this stuff, we’re being as smart as possible.”

Neither MLS nor the players’ union compiles data on specific injuries, such as concussions, but it’s hard to imagine any team with a darker history. Concussions contributed to the premature retirements of former United players Alecko Eskandarian, Josh Gros, Bryan Namoff, Devon McTavish and, in each of the past two offseasons, Davy Arnaud and Chris Rolfe respectively.

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“Unfortunately, we have too much experience, but I’m very confident in our staff and doctors that we’re doing the right things and making sure we’re putting best interests first,” Olsen said. “That means, at times, they are out a little longer than they could be.”

United’s head injuries have occurred in a wide variety of ways: in practices and games, while heading the ball, getting hit by a shot or pass, head-to-head collisions, with elbows or shoulders, and through a whiplash effect.

One allegedly didn’t happen on the field at all: Charlie Horton, a backup goalkeeper in 2016, sued teammate Fabian Espindola and the team, claiming the Argentine forward became angry and elbowed him on the head in the locker room following a film session. The case remains active in federal court.

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Namoff’s civil action against the team, claiming negligence in treatment of a 2009 concussion, was dismissed in 2014.

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Birnbaum suffered a concussion in the spring when the back of a New York Red Bulls player’s head crashed into his temple. He missed three matches.

The latest concussion came after heading the ball — the first off the top of his head while moving alongside an attacker, the second directly on the forehead.

With a second concussion in less than six months, “You think about it,” Birnbaum, 26, said of long-term implications. “I’ve still got a lot of time left to play, so I have to be careful about it.”

Brown, 26, suffered his injury Aug. 13, just a few minutes after a suspended match against Real Salt Lake resumed a day later at RFK. He collided with a defender in the attacking end and left the game. He is available for this week’s matches, Olsen said.

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Nyarko’s injury has raised the greatest concern because he has suffered numerous head injuries in his 10-year pro career. The latest setback occurred last month when he and Houston’s Ricardo Clark challenged for an aerial ball near midfield.

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While leaping, Clark extended his forearm and elbow. He hadn’t swung his arm, an act that has caused significant injury to soccer players, but did make contact with Nyarko’s head.

To make matters worse, as Clark fell, the bottom of his right foot raked Nyarko in the left eye.

“I was in real pain,” Nyarko said. “My eye started swelling and was irritated.”

He was evaluated for a possible concussion on the field — it was negative, the team said — and returned to the match.

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Nyarko complained of blurred vision from the contact by Clark’s foot and wasn’t able to practice. About a week later, both he and the team said, he began to have symptoms consistent with a concussion and was retested. He was placed into protocol.

As of Tuesday, Olsen said, Nyarko hadn’t returned to full training. It’s unclear whether he’ll play again this season.

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Nyarko, 31, wasn’t available to comment Tuesday, but a few weeks ago, he said: “With every hit, I take measures to make sure I heal before I come back. I am very diligent. Not to say it’s not a concern because you don’t know where the next hit is coming from.”

He said he is well aware of a recent study showing all but one of 111 former NFL players showed signs of chronic traumatic encephalopathy, a neurodegenerative disease.

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“It’s scary,” he said. “I’ve talked to specialists. It’s tough to gauge. They’ve talked about how someone with one or two concussions gets CTE and others with 10 concussions haven’t gotten it. You trust they do their due diligence.”

When word of Nyarko’s latest concussion began to circulate, he heard from Rolfe, his former teammate in both Chicago and Washington. Unable to recover from a head injury suffered early in the 2016 season, Rolfe last winter left the D.C. area and began pursuing career options outside of soccer. He remains on United’s roster because he has a guaranteed contract, but his playing days are over.

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“Chris told me he’s going to come rip up my contract to try to get me to stop playing,” Nyarko said. “He’s concerned and protective.”

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United notes: Hungarian midfielder Zoltan Stieber received a work visa and will likely make his debut in a reserve role Wednesday or Saturday. … Defender Taylor Kemp (hip) and midfielder Rob Vincent remain out, but midfielder Ian Harkes (ankle) has returned to full training. … United won the first two meetings with expansion Atlanta by a 5-2 count; D.C. has scored 15 goals in 23 matches against the rest of the league.

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D.C. United vs. Atlanta United

Where: RFK Stadium.

When: Wednesday at 7:30 p.m. ET.

TV: NewsChannel 8.

Records: D.C. 6-15-4, 22 points; Atlanta 10-7-5, 35 points.

D.C. probable starters: GK Bill Hamid; Ds Chris Odoi-Atsem, Sean Franklin, Kofi Opare, Nick DeLeon; MFs Paul Arriola, Marcelo Sarvas, Russell Canouse, Luciano Acosta, Lloyd Sam; F Patrick Mullins.

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Atlanta probable starters: GK Brad Guzan; Ds Anton Walkes, Michael Parkhurst, Leandro Gonzalez Pirez, Mikey Ambrose; MFs Julian Gressel, Jeff Larentowicz, Carlos Carmona, Miguel Almiron, Yamil Asad; F Hector Villalba.