Chris Pontius, left, and Taylor Kemp track ball in first half at RFK Stadium. (By Tony Quinn)

D.C. United Coach Ben Olsen pinned the ludicrous margin of defeat Saturday on playmaker Luciano Acosta’s red card early in the second half. Undermanned and out of sorts, United couldn’t keep up with the Philadelphia Union and endured a 4-0 loss, matching the worst home setback in club history.

But even before the ejection, which occurred with a one-goal deficit, United did not look good. After it, Olsen’s gang was hopeless.

And for the second consecutive weekend, United (3-5-2) lost at RFK Stadium to a one-win foe and didn’t score a goal. Through six home dates, D.C. has earned seven of a possible 18 points and gone scoreless four times.

“It’s alarm bells, for sure,” striker Patrick Mullins said. “It’s not going to make or break our season unless we let it. We have to respond.”

Olsen was not as concerned, saying: “I will let you know when it’s panic time. We’re certainly very far from that.”

Still, there are issues. United fell behind for the fifth consecutive game, this time on Haris Medunjanin’s hellacious shot from distance in the 39th minute before an announced crowd of 14,576.

With and without Acosta, United manufactured few quality chances. During the player shortage, D.C. was beaten on a set piece, counterattack and a sensational strike by Fabian Herbers.

Philadelphia (2-4-4) avenged a 2-1 away defeat to United six weeks earlier and won its second straight.

With Mullins, Steve Birnbaum and Bill Hamid returning to the lineup, United seemed poised for a good night. But it needed more than 30 minutes to find its way, and even then, opportunities were fleeting.

After dodging several threats, United fell behind. Chris Pontius squared the ball to Medunjanin, who smashed a 30-yard bid with venom, velocity and movement. Hamid failed to react fast enough and saw the wicked bid flash past him in an instant.

Hamid prevented further damage three minutes into the second half, slapping away Fafa Picault’s close-range header. A minute later, referee Chris Penso declined to award a penalty kick to United when Lloyd Sam was taken down.

United’s attack plunged deeper into darkness in the 55th minute when Acosta was sent off.

After 6 feet 4, 210-pound Oguchi Onyewu barreled him over from behind without a call, the 5-3, 130-pound Acosta took a wild swipe at Union midfielder Ilsinho. Penso reached to his back pocket. Acosta went face to face with the official. He will miss next Saturday’s match against the Chicago Fire.

Olsen said he didn’t disagree with the call but added: “I don’t think [Penso] had a pulse of the game. He was very inconsistent and that can lead to frustration. That’s not an excuse for Lucho to whack a guy.”

Although the deficit was one goal, there didn’t seem to be much hope. And in the 64th minute, Onyewu, a Washington native and two-time World Cup starter, extended the lead by shaking off Bobby Boswell’s mark and angling in a header off Medunjanin’s corner kick.

On his 35th birthday, the oft-injured Onyewu celebrated his first goal in league play since early 2012 when he was with Sporting Lisbon and first in any competition since the 2013-14 FA Cup with Sheffield Wednesday.

United stirred a bit but wasn’t good enough to make a game of it.

Philadelphia added a third in the 78th minute. After Sarvas surrendered the ball in midfield, Medunjanin picked out Picault for a bending bid from 18 yards. Ten minutes later, Herbers cracked a 19-yard one-timer.

“Some of it is because we’ve had a rotating cast, but that still shouldn’t happen at the magnitude it did,” defender Taylor Kemp said. “We’ve got to figure out a way to get things better.”

United notes: Defender Chris Korb made his first appearance since tearing a knee ligament against New York City FC on Aug. 13, 2015. He entered in the 55th minute for Kofi Opare.