Medical staff carries Steve Birnbaum off the field at Red Bull Arena on April 15. (Elsa/Getty Images)

D.C. United’s Steve Birnbaum took two knocks to the head last year, resulting in blood loss but no concussions.

Over four college seasons and three pro campaigns, despite numerous aerial challenges and hazardous headers in a rugged position, the center back had not missed any time because of severe head trauma.

His good fortune expired two weeks ago. Consequently, United’s captain will sit out his second consecutive match Sunday at Atlanta United and probably miss next Saturday’s home game against the Montreal Impact.

“Concussions are scary,” he said Friday in his first interview since the incident. “You start to think: ‘What if I don’t get better?’ But I’ve been pretty lucky — and got a hard head, I guess.”

The injury occurred April 15 in the waning moments of a 2-0 defeat to the New York Red Bulls in Harrison, N.J.

Birnbaum had contested a high ball in United’s attacking end, rising between two opponents. One smashed the back of his head into Birnbaum’s right temple. Down he went, crumpled on the grass, motionless, out cold for a moment.

“I remember I was sandwiched,” he said. “I was out for a second, then came to when the trainers got there. I remember getting taken off.”

He was treated on the field, carted off on a stretcher and taken to the locker room, then to a New Jersey hospital for a CT scan and further examination. Tests showed a concussion but no structural damage or other issues.

The team, as scheduled, returned home that night. Birnbaum retreated to the team hotel with athletic trainer Brian Goodstein. The next day, he rode Amtrak back to Washington.

For a week afterward, Birnbaum was shut down physically. He said he spend a lot of time on the couch and slept a lot while at home near Logan Circle. Some concussed players are ultrasensitive to light; Birnbaum didn’t have that problem. So he walked around his Northwest neighborhood.

He saw a specialist in the area. Symptoms began to subside. This week, he moved from riding a stationary bike to an elliptical, jogging at the training field and resuming light ball work.

“He’s moving forward,” Coach Ben Olsen said. “He’s been outside the last [few] days. It’s exciting news, not from a soccer standpoint, but just that he’s symptom-free and moving in the right direction.”

Birnbaum must still pass mandatory concussion protocol before re-engaging in extensive workouts. Given his progress, he should be able to ramp it up this coming week.

“I feel lucky it wasn’t worse than it seemed to be when it first happened,” he said. “It took me a little while to get over it. For a few days, I wasn’t feeling very good. I turned the corner pretty quickly.”

Last year, Birnbaum cracked heads in the season opener against the Los Angeles Galaxy and needed stitches but tests showed he did not suffer a concussion. The next match, he was back in the starting lineup.

Late in the year, while on U.S. national team duty in Cuba, he got kicked in the head, opening a wound. The U.S. medical staff administered tests but did not detect a concussion. He was back in United’s lineup a week later.

The recent injury, however, felt different.

“When it happened, I was super dizzy and seeing double,” he said. “I couldn’t really keep my balance.”

Birnbaum had become the latest in a long line of United players to miss time with concussions.

Last year, midfielder Patrick Nyarko suffered two during the season. Chris Rolfe, the 2015 team MVP, has not played since his concussion early last season. And although he remains on United’s roster with a guaranteed contract, he has all but retired. He left the D.C. area and is pursuing career opportunities outside of soccer.

In the past 10 years, concussions prompted former United starters Alecko Eskandarian, Josh Gros, Bryan Namoff, Devon McTavish and Davy Arnaud to retire early.

“I try not to think about it,” Birnbaum said of the team’s concussion history. “I feel like I’ve been fortunate not to have too many in my lifetime, especially in a close time period.

“I’ve been very careful with what we’ve been doing in my recovery. That’s been the biggest thing: Knowing my body and myself, not pushing it too hard and coming back in the right way.”

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D.C. United at Atlanta United

Where: Bobby Dodd Stadium.

When: Sunday, 3 p.m.

TV: FS1, Fox Deportes.

Records: D.C. 2-3-2, 8 points; Atlanta 3-2-2, 11 points.

D.C. probable starters: GK Bill Hamid; Ds Sean Franklin, Kofi Opare, Bobby Boswell, Taylor Kemp; MFs Lloyd Sam, Jared Jeffrey, Ian Harkes, Luciano Acosta, Lamar Neagle; F Sebastien Le Toux.

Atlanta probable starters: GK Alec Kann; Ds Tyrone Mears, Michael Parkhurst, Leandro Gonzalez Perez, Greg Garza; MFs Hector Villalba, Jeff Larentowicz, Carlos Carmona, Miguel Almiron, Yamil Asad; F Kenwyne Jones.