Los Angeles FC forward Diego Rossi gets the ball past D.C. United goalkeeper Bill Hamid for one of his three goals Saturday at Audi Field. (Toni L. Sandys/The Washington Post)

After video review ended Wayne Rooney’s afternoon with a deserving red card Saturday, a dismissal that punctuated a 4-0 stinker against Los Angeles FC, D.C. United’s English star marched some 60 yards without interruption to the stairs leading to the locker room below Audi Field’s south stands.

Rooney, at least, got to escape the carnage. His teammates were not as fortunate, embarrassed from start to finish on a sun-splashed day that exposed the massive gap between MLS’s best team and its supposed second-best side.

Unbeaten in the regular season since last summer and scored upon just once through four matches this spring, United conceded three goals in the first 32 minutes and showed no signs of the team that was poised for the best start in club history.

“It’s important to learn that, just because you are at home, just because you are in the comforts of Audi Field, the game is not just going to come easy and we’re going to dominate teams,” said Ben Olsen, the team’s visibly angry coach, after United (3-1-1) lost a regular season match for the first time since August.

Los Angeles, a second-year operation, improved its league-best record to 5-0-1 and increased its goal total to nine over its past two weekends and 19 overall. Uruguayan Diego Rossi recorded a hat trick, and Mexican Carlos Vela increased his MLS-leading haul with his seventh goal, offsetting a failed penalty kick in the early moments.

“Our pulse was wrong from the get-go,” Olsen said. “That part is concerning. I will look to myself first because you prepare all week tactically . . . but it’s also still about some of the old-school sayings that you have to show up every game and play with a certain edge and certain desperation.”

United’s first deficit of the season turned into a stunning chasm well before halftime, and with the Montreal Impact visiting Tuesday, it will force the team to reevaluate matters in short order.

“It didn’t seem we were up for the game, especially the first 30 minutes,” defender Steve Birnbaum said. “We’re going to have to respond Tuesday. We’ve got to respond in a big way.”

United will have to do it without Rooney, the captain and the team’s scoring leader, who will miss a match for the first time since he arrived last summer. MLS’s disciplinary committee could impose a longer suspension as well.

The incident occurred in the 52nd minute, when Rooney lunged at Rossi with his right foot and caught the attacker in the lower left leg. Initially, referee Robert Sibiga assessed a yellow card but, after reviewing the replay, changed it to a red.

“I have no arguments," Rooney said. "It wasn’t malicious; I didn’t try to hurt him. But obviously I caught him. I was expecting [a red card] really when the referee gave me the yellow. It’s the right call.”

The game was lost long before his departure. D.C. giveaways left the defense misshapen and exposed, and they allowed LAFC to unleash a lethal transition.

United did not have enough pace in the back to corral Rossi and Vela, who were threats almost every time they touched the ball in the open field before an announced sellout of 20,600.

“I am not concerned about the pace,” Olsen said. "I am concerned about our decision-making and how much space we give opponents. We didn’t do a good job in transition, understanding where the dangerous space was.”

Los Angeles exploited space and seemed capable of scoring five or six in the first half.

United’s problems began with Leonardo Jara’s handball in the box. Bill Hamid bailed out his team by stopping a penalty kick by Vela, who, on the rebound, hit the right post.

The reprieve was brief. A miscommunication in the midfield between Jara and Russell Canouse presented space to concoct another opportunity. Adama Diomande played the ball to Vela, who cut back on Birnbaum and curled an 18-yarder into the left corner in the 15th minute.

United cracked under increasing pressure. Vela glided on the right side and delivered a wonderful ball to Rossi, who was making a run between center backs Birnbaum and Frederic Brillant. Rossi’s one-time stab beat Hamid to the near corner in the 27th minute.

Even with Diomande exiting with a hamstring injury, LAFC did as it pleased. United labored to sustain possession and create quality chances. Playmaker Luciano Acosta was bottled up, and Rooney received little service.

Five minutes after his first goal, Rossi struck again, drifting free on the left side after Paul Arriola’s giveaway. Mark-Anthony Kaye set up Rossi, who picked out the far corner beyond Hamid’s reach.

If hope was not completely lost, it fizzled seven minutes after intermission when Rooney was sent off. Rossi’s composed finish in the 76th minute added to United’s misery and secured D.C.'s third defeat in 18 matches since the Buzzard Point venue opened.

Playing again Tuesday, Arriola said, is “a great way to forget about this game — look forward and continue in the right direction.”