WASHINGTON -- Long after Fredy Montero's matchwinner flashed past goalkeeper Josh Wicks, a mood of shock and distress lingered over RFK Stadium on Saturday night as D.C. United contemplated several sickening realities in the wake of their 2-1 loss to Seattle Sounders FC.The goal brought about United's first home loss of their 2009 MLS campaign. It also landed a significant blow to their playoff hopes, which remain all too uncertain with just four league matches left to play.And perhaps most gallingly, it represented a second home humiliation at the hands of Sounders FC in the past 10 days, Seattle having snatched the U.S. Open Cup title from United on Sept. 2, also by a score of 2-1."Definitely, we're disappointed that they came to our house and beat us twice. So I think also we've got to give them credit," said United captain Jaime Moreno afterwards, sitting alone in a particularly gloomy home locker room. "It's a big lesson for us -- at this point we just can't make mistakes, overall."D.C.'s defense had been gashed open by Steve Zakuani's opener just four minutes after the opening whistle, but United seemed to have gotten the measure of the visitors as they slowly but surely hauled themselves back into the match and eventually equalized through a Rodney Wallace strike in the 54th minute. A string of tantalizing chances to seize the lead soon followed and the home fans believed that a comeback win might be close at hand.Arriving as it did with just six minutes left on the clock, Montero's goal punctured all that, too."As soon as Rodney got the goal, I thought we were all over them. We had more of the possession, we were more dangerous," said rookie midfielder Chris Pontius. "We didn't put some balls away and they did at the end. It's just tough, especially at home, after they beat us in the Open Cup final. We wanted revenge tonight and we didn't get it on this night."D.C. had wasted several excellent scoring opportunities as the second half unfolded. But seconds before Montero's goal, United coach Tom Soehn made two substitutions intended to finally corral a late winner, throwing Christian Gomez and Andrew Jacobson into the fray.Gomez immediately sprinted over to James Riley to pressure Seattle's right back as he advanced down the right channel, only for Riley to cut inside onto his weaker left foot and stroke a savvy through ball into Montero's path.D.C. center back Julius James was close at hand, but he arrived a fraction of a second too late and only got the slightest of touches on the Colombian's sudden blast."Everything happened really fast," said United left back Avery John. "[Montero] got a quick turn, a quick release. We know that's one of his strong points, turning and shooting. I thought we could've been a little more aggressive in closing down the pass, closing down the shot. But Julius did close him down -- it hit his foot and deflected up, so that kind of threw everything off. It is disappointing. It's very hard, because we had our chances to kill them."The Kansas City Wizards might approve of the harsh turn of events. K.C. carved out plenty of scoring chances in their 1-0 loss to United at RFK on Wednesday night, but lost all three points to a more opportunistic D.C. side that made the most of a Luciano Emilio goal.Saturday saw the roles reversed in acutely painful fashion for the Black-and-Red."That's what happens. We've got to put our chances away. This is part of the game," said Moreno. "So we know that, and we know that we have so many important games coming up and we can't afford to lose any more points at home. ... I don't think we were sharp enough."