Steve Birnbaum tracks Red Bulls forward Bradley Wright-Phillips on March 22. (John Minchillo — Associated Press)

D.C. United defender Steve Birnbaum will miss four to six weeks with a sprained left ankle suffered early in Saturday’s 1-0 victory over the Los Angeles Galaxy.

Birnbaum, a second-year starting center back who received a U.S. national team call-up in January, landed awkwardly after heading a corner kick on target. He was removed on a stretcher and underwent an MRI exam Monday.

The injury ended a streak of 2,075 consecutive minutes in MLS’s regular season, a run that began last June.

Kofi Opare replaced Birnbaum on Saturday and is slated to partner with captain Bobby Boswell in central defense for the foreseeable future, starting Friday night at Orlando City. If not for the injury, Birnbaum probably would have joined the U.S. squad for the April 15 friendly against Mexico in San Antonio.

“It’s not good news about Steve, but I am at the point with injuries that it’s just another chance to see our depth and see who is ready to come in and who is not, for now and the future,” Coach Ben Olsen said. “It helps to have the right pieces that you have faith in.”

Birnbaum’s setback adds to the list of unavailable regulars: Forwards Fabian Espindola (six-game suspension), Luis Silva (hamstring) and Eddie Johnson (heart condition), as well as midfielder-defender Markus Halsti (knee), have yet to play this season.

Opare and Boswell are familiar with one another’s style: While Birnbaum was away at U.S. camp, they were Olsen’s primary tandem during the first several weeks of preseason in Florida.

“They got minutes together, and for that central partnership, putting shifts in is a big deal,” Olsen said. “It’s Kofi’s turn to help this team for a while.”

With Opare thrust into the lineup and Halsti (MCL sprain) out for a few more weeks, United lacks depth at center back. Olsen said, if necessary, he would turn to defensive midfielder Perry Kitchen. “He has played there before. It’s not his ideal spot, but we could move him back.”

Even when healthy, Halsti will need time to regain fitness.

In a lighter moment, Olsen, a former midfielder, cracked wise about the fitness demands on center backs: “Come on, what do they really do during the game? How fit do you have to be to play center back?”

Another option to provide depth is Jalen Robinson, who is playing center back on loan with the third-division Richmond Kickers. Robinson has no regular first-team experience but might receive a call-up to serve on the game-day roster.

This weekend, though, outside back Chris Korb will be the only available substitute defender.

Meantime, Silva has made steady progress in his recovery and seems likely to suit up Friday in Florida.

Midfielder Collin Martin, who was assigned to Richmond along with Robinson, goalkeeper Travis Worra and outside back Luke Mishu, might have to have groin-hernia surgery, Olsen said.

Reflecting on Saturday’s last-minute victory, which came six days after a poor showing at New York, Olsen said: “It was a good response. Our energy level was better. There is still some stuff to iron out, but when it’s not perfect, we find a way to get a result. It was a fairly even game: They had looks, we had looks. Neither team really was that great in the final third. So we’ve got to get better there. But as a foundation, we need that aggressive mind-set from our midfield to our backs. That has to be our baseline and identity.”

On Monday, when United returned to practice, it crossed paths at RFK Stadium with the Argentine national team. Lionel Messi and Co. moved from Georgetown to RFK for the final two days of workouts before heading to New Jersey for tonight’s friendly against Ecuador.

Olsen caught up with Albiceleste Coach Gerardo Martino, whom he first met during a scouting mission in Argentina a few offseasons ago.

At the time, the DCU staff was eyeing Newell’s Old Boys forward Maxi Urruti. Martino was Newell’s head coach. (Urruti ended up signing with Toronto and is now with Portland.)

“The day before Newell’s match, Martino sat us down and took us through his game plan for 15 minutes,” Olsen said. “Really sweet guy, a real gentleman.”