MIRED IN a season that has fallen well short of expectations, the Union fans who made the trek to PPL Park Thursday night were given a rare treat. They got to see Lionard Pajoy, the team's joint leading scorer coming into the game, in the starting lineup.

There was a slight problem, though: Pajoy was starting for the other team.

It's been that kind of year for Union fans. Pajoy has been on D.C. United's roster since the middle of last month, when the Union traded him to a team that is in the thick of the playoff race.

Pajoy hasn't done much better in D.C. than he did here, with only one goal in six games since the move. But it seemed to surprise few in the announced crowd of 16,024 when Pajoy scored the goal that dealt the Union a 1-0 loss.

"It's very frustrating," Union captain and fellow Colombian Carlos Valdés said afterward. "We know the way Pajoy plays. We lost focus for 1 minute - 1 second - and he took advantage."

Union manager John Hackworth suggested in his midweek news conference that he intended to spice up the offense, and he stayed true to his word. He gave Princeton product Antoine Hoppenot a rare start on the forward line, alongside regulars Jack McInerney and Danny Cruz.

But the hoped-for spark failed to ignite. The first half ended scoreless, with the Union creating only a few decent looks and half-chances.

One of the better opportunities came from a goal-mouth scramble in the 18th minute after United goalkeeper Bill Hamid spilled a corner kick. The ball bounced free in the 18-yard box, but none of the four Union players who converged on it was able to take a shot.

D.C.'s best chance came in the 23rd minute, when Chris Pontius smashed a 35-yard free kick that Zac MacMath could only parry away. That was the only official shot on goal registered by either team in the first 45 minutes.

The second half had a livelier start, as the Union forced two saves from Hamid in the first 5 minutes.

McInerney jolted a subdued River End in the 52nd minute, taking out his frustrations by rocketing a low shot on frame from 30 yards. Hamid couldn't handle the bouncing ball, and had to dive to parry it away from an onrushing Hoppenot. McInerney threw up his hands in frustration as the chance went begging.

Hackworth was left to ponder yet another lost opportunity.

"A little bit of me feels like we can't buy [a goal], no matter what," he said, later adding that his young forwards "haven't learned how to put the ball in the back of the net when it really matters."

In the 67th minute, the Union's lack of punch finally came home to roost, as D.C. took the lead on a textbook counterattack.

Maicon Santos smacked a long ball to Pontius, who deftly trapped the ball on the right flank and lured MacMath off his goal line. Pontius then floated a pass over MacMath to Pajoy, who volleyed the ball into an open net.

Going down a goal forced Hackworth's hand. Having already replaced Hoppenot with forward Josué Martinez, Hackworth pulled defender Gabriel Farfan and sent on another forward in Chandler Hoffman. But, as with Martinez, the Union's top draft pick this year didn't have much of an impact.

So in the 77th minute, Hackworth fired the biggest bullet he had left in the proverbial revolver. In came Freddy Adu, replacing Gabriel Gómez.

But Adu didn't help much, either. The best he could offer was a shot well over the crossbar from 20 yards in the 84th minute, set up with a few trademark stepovers.

Adu is well aware of the pressure he faces as the Union's highest-paid player. He freely acknowledged that he is playing now to remain with the team next year.

"I've got to earn my spot like anybody else," Adu said. "I love this team, I love this organization and I want to be a part of [it] going into the future."

The last, best chance for a goal fell to Cruz in second-half stoppage time. He corralled the ball as it pingponged around Hamid's box, but couldn't get a shot off after he turned to face the net.

When Cruz, Adu and their teammates entered their locker room after the final whistle, they passed a sign that read "Celebrate The Struggle."

On a night like this - indeed, in a season like this - there wasn't much else left.

Contact Jonathan Tannenwald at jtannenwald@phillynews.com. Follow him on Twitter @thegoalkeeper.