Sir Alex Ferguson felt inexperience was to blame for Manchester United's surprise defeat by 10-man Kansas City Wizards.
Kei Kamara scored the Wizards' winner in the 42nd minute as the Major League Soccer side excelled in front of 52,424 fans at the Arrowhead Stadium.
Dimitar Berbatov had pulled United level a minute earlier from the penalty spot after Davy Arnaud had opened the scoring for the hosts in the 11th minute.
Although Jimmy Conrad was sent off for the tackle on Berbatov that led to the spot-kick, the Wizards stayed resilient and achieved a notable victory.
It was United's first defeat on their North American tour following victories against Celtic and Philadelphia Union.
Ferguson fielded a young defence from the start, with Ben Amos in goal and Rafael, Chris Smalling, Jonny Evans and Ritchie De Laet making up the back four.
Ferguson said: "I thought the inexperience of the defenders showed in the first 15 minutes. We never quite recovered from that.
"That was a bit of a handicap, that inexperience.
"I felt our young goalkeeper organised them and calmed them down. Their centre forward [Teal Bunbury] was a handful physically. He gave us a hard time that way.
"They are very committed, the American players, they are very athletic. It is a big game for them. They are playing Manchester United.
"You have to expect that kind of motivation. You know they are going to get stuck in. I wasn't too bothered about that to be honest."
United stepped up the pressure in the second half but striker Mame Biram Diouf squandered a couple of good chances.
Ferguson said: "We had two good early ones from Diouf just after half-time which would have swung the game in our favour.
"Our possession of the ball today was ragged at times. Normally we are much better. That will come hopefully with the games."
The one consolation for United was Nani's 69-minute outing on his return from a dislocated collarbone, although Ferguson admitted the player was off the pace after the break.
Ferguson said: "Maybe we should have taken him off at half-time because we did get anything from him in the second half. Sometimes it goes that way when a player has been out for a while.
"He has only had seven days' training, so maybe we should have taken him off at half-time. It is good to have him back nonetheless."
Meanwhile, Darren Fletcher and John O'Shea are almost certainly out of the match against the MLS All-Stars in Houston on Wednesday after suffering "niggling injuries".