Though other trades brought bigger headlines, United’s acquisition of Brandon McDonald has brought the vocal leadership it needs in the locker room. (Ned Dishman/GETTY IMAGES)

For three months of the MLS season, D.C. United’s novice back line had proved its capacity but lacked bark and bite. Too often, communication lapses and gentle resistance yielded soft goals, turning plausible victories into maddening draws and gutting losses.

Needing an assertive presence, United went shopping around the league and, three weeks ago, obtained Brandon McDonald from the San Jose Earthquakes in exchange for financial considerations.

“He’s got a little nasty in him,” Coach Ben Olsen said. “It was much-needed back there.”

McDonald, 25, is expected to make his third consecutive start Saturday night when United (5-5-7) visits FC Dallas (10-5-4). Last weekend, with an animated and imposing McDonald in charge, United extended its unbeaten streak on the road to four by blanking Thierry Henry and the New York Red Bulls, MLS’s top-scoring player and club, respectively.

“Ben said that’s one of the reasons he brought me in here,” McDonald said, “so that’s my job to get it done.”

McDonald’s influence has steadied a defensive corps that includes rookie backs Ethan White, 20, and Perry Kitchen, 19, and goalkeeper Bill Hamid, 20. His acquisition also helped compensate for long-term injuries to veterans Dejan Jakovic, Jed Zayner and Marc Burch.

Jakovic, the club’s most experienced central defender, is back in the mix after missing six weeks with a hamstring injury, but the McDonald-White partnership might have lasting power. At New York, White seemed to draw inspiration from his new mentor.

“You always know where he’s going to be, where he is going to go, where he wants you,” the former Maryland Terrapin said. “It makes everyone’s job easier.”

McDonald is in his fourth season out of the University of San Francisco. Selected by Los Angeles in the fourth roundof the 2008 MLS draft, the 46th of 56 picks, he lasted only one season with the Galaxy.

Signed by San Jose, McDonald started 18 league matches in 2009 and was invited to a U.S. national team training camp featuring mostly MLS players. Last season, he started all but two games in helping the Earthquakes advance to the conference final.

After beginning this season in the San Jose lineup, he lost his starting job. Smitten with McDonald’s size (6 feet 1, 185 pounds), leadership and experience, United reached out to the Earthquakes.

McDonald’s arrival was overshadowed by United’s acquisition the same day of star midfielder Dwayne De Rosario from the Red Bulls.

“We need a little personality,” Olsen said. “We have talent, but we need a personality. Dwayne and Brandon have brought it. When you have that, it brings out personalities in everyone else.”

McDonald leads with his actions and voice.

“You can hear him all over the field,” Pontius said. “You don’t want to not do what he says because he’ll get on you. He holds you accountable.”

McDonald isn’t the only soccer player in the family. His sister Jessica was a starting forward on NCAA championship teams at North Carolina in 2008 and ’09, contributing 14 goals and 16 assists overall. In her final season, she scored the lone goal in the title game against previously unbeaten Stanford. That UNC squad included midfielder Tobin Heath, a member of the U.S. national team at the Women’s World Cup in Germany this summer.

Jessica, 23, played for the defunct Chicago Red Stars in Women’s Professional Soccer last year before she injured a knee. She hopes to resume her career soon.

As children in Glendale, Ariz., Brandon, Jessica and two cousins played regularly. She became a basketball, soccer and track and field standout; he excelled in soccer and played for the elite Sereno Golden Eagles youth club, which also produced 2010 World Cup forward Robbie Findley.

In college, Brandon transitioned to defense after playing forward for years. Many players resist such a move; he embraced it.

“Coaches would tell me I’d have to be versatile if I wanted to get more chances down the road,” he said. “I had an open mind to it. I love it. It’s a position you can be other people’s eyes. You can communicate with them, control the system you are in. I enjoy fitting into a system.”

How long he’ll stay in United’s system, however, is unclear. McDonald is in the final year of his contract and might test the European waters after the season.

“I have had offers [in recent years] to stay in the league longer, but I’m waiting it out and I’ll see what happens,” he said. “We have a good team here to push for the playoffs. I’m just worried about this season. We can do some things.”