Celtic were comprehensively outplayed as they continued their pre-season build-up in the United States. DC United put four past the Scottish champions, with 17-year-old Freddy Adu getting the ball rolling on half-time. Jamil Walker doubled the Major League Soccer team's advantage and Celtic had another setback when Kenny Miller was dismissed for a foul on John Wilson. Bobby Boswell and Andy Metcalf added further goals after the 65th-minute red card in Washington. Celtic manager Gordon Strachan said his players need to step up their stamina levels after watching them subside in the second half. "They blew us off the pitch with their energy, especially in the second half," he conceded. "As the game went on, our players felt less energy. That is disappointing. "If you are lethargic, you will give chances away. If you cure that then the other things will be cured. We need to work on the lethargy problem." The defeat was Celtic's third successive friendly reverse following a two-match tour of Poland. Next up for the Bhoys is an outing against Chivas Guadalajara at Giants Stadium on Friday in East Rutherford, New Jersey.