Ruben Amorim’s side, who finished 15th in the Premier League for their worst season since 1974, looked lethargic and devoid of ideas in the gruelling heat.
Some fans at Bukit Jalil National Stadium left before the end and there were boos at the final whistle.
United have struggled for goals all year and it was more evidence why they are reportedly prepared to shell out US$85 million on Brazilian international attacker Matheus Cunha from Wolves.
Amorim, who was clearly frustrated during the match and shook his head at times, refused to confirm the Cunha deal.
The Portuguese coach was happy though to leave the Malaysian capital with no new injuries and had no issue with fans booing his team off.
“We were slow and we still don’t have it in us to win every exercise in training, every game,” he said. “The boos from the fans, it’s something that we need.”
“These people believe in Manchester United no matter what the context. It’s our duty to improve the team and to improve the team next season,” he added. “For me, we need to have that belief, that hunger to win every game, that is important.”
In their first of two post-season games in Asia, United started with several regulars including Harry Maguire, Rasmus Hojlund, Casemiro, Diogo Dalot, Manuel Ugarte, Kobbie Mainoo and Andre Onana.
In temperatures of 32 degrees Celsius (90 Fahrenheit) at kickoff, it was the Asean All Stars who looked more threatening in the first half.
They shot just wide in the 16th minute and Onana was then called into action with a low save that endeared him to the pro-United crowd.
Mainoo provided the first real threat for United with a 25-yard effort that was turned behind for a corner, before the young midfielder saw his shot cleared off the line in the 38th minute.
Captain Bruno Fernandes came on in the second half and his half-volley lit up a drab restart.
Argentine winger Alejandro Garnacho, on the trip despite looking like he is headed for the exit at United, made a lively cameo off the bench.
United were better in the second half but the Southeast Asian side stunned the visitors in the 71st minute when Burmese attacker Maung Maung Lwin found himself clean through on goal before firing into the top corner past Tom Heaton.
Soon after Heaton came to United’s rescue to prevent further embarrassment.
United play the Hong Kong, China team on Friday. (AFP)