United goalkeeper Bill Hamid, in green, cannot keep a headed shot by Houston rookie Will Bruin out of the net. Houston won, 4-1; Bruin scored three goals. (Bob Levey/GETTY IMAGES)

First came word of a serious injury to a key player and, as a breezy evening at Robertson Stadium progressed Friday, D.C. United’s outlook grew increasingly worse.

After revealing that midfielder Branko Boskovic had suffered a torn knee ligament earlier in the week, United put forth another dreadful defensive effort and was pummeled by the Houston Dynamo, 4-1, in front of 20,577 witnesses.

“It was men versus boys,” Coach Ben Olsen said. “We lost every matchup on the field. . . . We weren’t good enough in any aspect of soccer.”

United (2-4-1) has conceded eight goals in the past two league games and 11 in three matches in all competitions. Rookie forward Will Bruin scored a hat trick for the Dynamo (3-1-3), which ran its unbeaten streak to six and improved to 5-0-1 all-time at home against United.

D.C. defender Marc Burch evened the match in the first half, but Bruin answered with his second two minutes later and the Dynamo pulled away after intermission.

“This is schoolyard stuff, this is little kids’ stuff,” said captain Dax McCarty, who blamed himself and the veterans first. “It’s just unacceptable.”

Reflecting on the loss and Boskovic’s injury, McCarty added, “It’s just worse news on top of the feeling of getting your [butt] kicked.”

Boskovic, United’s most influential player the previous two matches after a sluggish start, was injured during a 3-2 loss to the New England Revolution on Tuesday in a U.S. Open Cup qualifier. He had scored twice in a late nine-minute span and endured a number of hard tackles, the last of which knocked him from the game.

X-rays Wednesday didn’t reveal anything serious, and there was thought of him accompanying the team to Texas. He had an MRI exam Thursday and, as a precaution, skipped the trip. Moments before kickoff Friday, United announced that he had suffered extensive damage.

Boskovic plans to seek a second opinion, but if the same diagnosis were reached, he would undergo surgery and miss four to six months.

Boskovic’s absence will also be felt in Europe. He starts for Montenegro’s national team, which is in a tight race with group favorite England for a berth in the 2012 European Championship.

Except for the return of right back Jed Zayner, Olsen fielded the same lineup that had lost to New York, 4-0, last week. United was competitive for a half and stirred for a bit after intermission, but overall was outclassed.

“We are a young team, but we’ve got to stop making that excuse for ourselves,” midfielder Clyde Simms said. “We’ve had some games together now and we’ve got to turn this around.”

The Dynamo wasted little time seizing on United’s instability. In the fourth minute, Geoff Cameron stepped in front of Simms on a back pass by McCarty. Cameron waited for Bruin to make a run before slotting the ball between Zayner and Dejan Jakovic. Barely onside, Bruin collected the ball in stride and beat goalkeeper Bill Hamid to the near corner.

United drew even in the 39th minute on a set piece about 26 yards away. Josh Wolff and McCarty touched the ball, leaving it for Burch, whose powerful drive skidded beyond Tally Hall’s reach and into the lower right corner.

The Dynamo quickly regained the lead — another devastating turn of events for United, which, early this month in Colorado, conceded a goal one minute after scoring a second-half equalizer.

Cameron’s pass led Brad Davis to the end line for a cross to the six-yard box. Perry Kitchen lost track of Bruin, who drove a header past the helpless Hamid.

Andy Najar entered at halftime and briefly lifted United, but in the 57th, Bruin rose above a pack of players at the edge of the six-yard box to nod in Davis’s free kick. Four minutes later, Weaver was left unmarked on Hunter Freeman’s throw-in and drilled a rising shot from a tight angle over Hamid.

“They beat us for 90 minutes,” Olsen said. “They have guys that get it. They have guys that get that soccer is about more than knocking the ball around and showing up 80 percent of the game.

“This is a job, and I don’t know if some of our guys understand the seriousness of what we’re trying to do. If it continues, we’ll have to make changes, real drastic changes.”