Manager Gordon Strachan admitted Celtic had been no match for DC United after the Hoops suffered a 4-0 defeat in the first game of their three-match tour of America at the RFK Stadium in Washington.
The match also featured a red card for new signing Kenny Miller as the Glasgow team were outplayed by their Major League Soccer counterparts to suffer their third consecutive friendly defeat following a two-game tour of Poland.
Strachan offered no excuses for the defeat admitting their opponents took advantage of their greater stamina.
'They blew us off the pitch with their energy,' admitted the Hoops boss. 'Especially in the second half. As the game went on, (our) players felt less energy. That is disappointing.
'It was the lethargy, really. If you're lethargic, you'll give chances away. If you cure that, then the other things will be cured. We need to work on the lethargy problem.'
Celtic's best chance in the first half came on the half hour when Jiri Jarosik sent a long shot over the bar.
Despite being outplayed for much of the first half, it appeared Celtic would be going into half-time on level terms but Freddy Adu, just 14 when he signed a professional deal with the MLS in 2003, struck in the 44th minute after being sent through by Christian Gomez.
As the Celtic defenders appealed for offside, the 17-year-old slotted the ball to the right of goalkeeper David Marshall.
Substitute Jamil Walker doubled the lead in the 57th minute before the Hoops were reduced to 10 men when Miller was sent off in the 65th minute for a tackle on John Wilson.
From the ensuing set-piece, Gomez put in a cross which Bobby Boswell headed past Marshall.
Andy Metcalf completed the rout in the 78th minute when he dribbled unopposed into the area and beat Marshall.
Strachan did not attempt to excuse Miller for his sending off, saying: 'We're all frustrated, you have to be able to handle it. Kenny has to accept whatever comes his way.
'It's disappointing for the Celtic fans who live in this area. They'd like to see their team play better. We'd all like to see them play a bit better.'
Celtic take on Chivas Guadalajara at Giants Stadium on Friday evening in East Rutherford, New Jersey.