KUALA LUMPUR: Manchester United manager Ruben Amorim cut a frustrated figure but remained resolute in his footballing principles after his side slumped to a surprise 1-0 defeat against the Asean All Stars in the Maybank Challenge Cup at the National Stadium in Bukit Jalil on Wednesday.

A solitary second-half strike by Myanmar's Maung Maung Lwin was enough to hand the Red Devils an unexpected loss, leaving the 72,550 fans in attendance audibly disgruntled at the final whistle.

Despite the setback and United's evident lack of cutting edge, Amorim insisted he would not abandon his tactical approach, even as he shouldered the blame for the underwhelming performance.

"Yeah, of course, we should have performed better," Amorim said in the post-match press conference.

"We were slow, and we still don't have it in us to win every exercise, every game. But the important thing is that we didn't have any injury."

The Portuguese manager, known for his possession-based attacking style with heavy emphasis on pressing that brought him success in his homeland, acknowledged the fans' frustration, particularly after a performance that saw United dominate possession but struggle to create clear-cut chances.

Asked about the boos that greeted the final whistle, Amorim didn't flinch. "I feel always guilty," he said.

"I said that in the last game, in the last game I'm always guilty of the performance of the team, no matter what. Since I am here from the first day, I am guilty."

He even suggested the fans' vocal disapproval could serve a purpose. "The boos from the fans, I think it's something that we need, maybe. Because every game that we lost in the Premier League, they were always there. It's like the way of playing, if they see that it is not working, they will change the way they behave."