D.C. United Coach Ben Olsen is happy about Steve Birnbaum’s learning curve since he began filling in for Jeff Parke. (Jonathan Newton/The Washington Post)

For three months, Steve Birnbaum watched and learned and waited. D.C. United had chosen the University of California center back with the second overall pick in the MLS draft, yet in need of backline experience, the club also acquired two seasoned central defenders over the winter.

So with Bobby Boswell and Jeff Parke forging an effective partnership, Birnbaum remained rooted to the bench.

It probably would have stayed that way throughout most of the campaign, except Parke hurt his left foot in early June.

Birnbaum went from never playing a minute to playing every minute — seven consecutive starts for a D.C. club that has won four straight and, with a victory at Houston on Sunday evening, would move atop the Eastern Conference.

Parke recovered from the injury but is sidelined indefinitely with migraines, leaving Birnbaum in a critical role as United (11-5-4) seeks to stay on playoff pace.

Despite Birnbaum’s inexperience, Coach Ben Olsen said he did not hesitate to call on him.

“It was excitement rather than trepidation,” he said. “We were seeing every day [in preseason and practice] the type of player and competitor he was. We wanted to test him in a real game, but Jeff and Bobby were doing so well, there just wasn’t an opportunity to change the center backs.”

With Boswell and Birnbaum bonding, Sean Franklin and Chris Korb performing well on the corners and goalkeeper Bill Hamid in top form, United has MLS’s third-best goals against average. Last year, with a mostly different defensive cast, it was second worst.

“At the beginning of the year, the coaches were just trying to get me into the groove of being a professional,” Birnbaum said. “It was nice to learn from Bobby and Jeff, and once I got my chance, I felt I was prepared. It was a good transition, watching a little bit and then getting my time.”

After five years at Cal, including one redshirt season, Birnbaum, 23, arrived more mature and polished than most rookies who left school early. That has shown on the field: He is comfortable with the ball and makes smart decisions.

“When you talk about rookies, you talk about young guys, but he is a little bit older,” said Boswell, in his 10th season. “He has a more experienced head on his shoulders than people realize. He learns a lot during the week, and he has learned not to get beat on the weekend.”

From the bench, Birnbaum gained an appreciation for the speed of both play and thought in the pros.

“The forwards’ movements in this league are so much better than in college, so I really watched Bobby and Jeff communicate and organize the back line,” he said. “Bobby is great talking through plays, so when I am on the field, I communicate through him and track the runners.”

Birnbaum’s lack of playing time early in the year has allowed him to remain fresh; this is the time of the year when rookies, accustomed to a short college season, face physical and mental exhaustion.

“I am hitting my season right now,” said Birnbaum, who, aside from the MLS matches, started in a U.S. Open Cup defeat at third-division Rochester. “I am feeling good and fresh and ready to take on August.”

This month will test not only Birnbaum but the entire squad, which, amid a stretch of eight matches in 33 days, has been weakened by injuries. With Parke out, United on Tuesday acquired second-year center back Kofi Opare from the Los Angeles Galaxy.

For now, though, the job belongs to Birnbaum.

“He has learned a few lessons that haven’t necessarily cost us and he will continue to learn them,” Olsen said. “He has a ways to go, but for him being a youngster and to step in and do what he is doing has been a big positive.”

MLS notes: Boswell and Franklin were added to the all-star squad, joining Hamid for Wednesday’s match against Bayern Munich in Portland, Ore. They replaced Seattle’s Chad Marshall and Los Angeles’s Omar Gonzalez. Philadelphia defender Maurice Edu will step in for Galaxy forward Robbie Keane.

D.C. United at Houston Dynamo

Where: BBVA Compass Stadium.

When: 8 p.m.

TV: Comcast SportsNet.

Radio: WJFK 106.7 FM.

Records: United 11-5-4, 37 points; Dynamo 5-11-4, 19 points.

D.C. probable starters: GK Bill Hamid; Ds Sean Franklin, Bobby Boswell, Steve Birnbaum, Chris Korb; MFs Nick DeLeon, Perry Kitchen, Davy Arnaud, Chris Rolfe; Fs Luis Silva, Eddie Johnson.

Houston probable starters: GK Tally Hall; Ds Kofi Sarkodie, Jermaine Taylor, David Horst, DaMarcus Beasley; MFs Oscar Boniek Garcia, Luis Garrido, Ricardo Clark, Brad Davis; Fs Giles Barnes, Will Bruin.