Manchester United manager Jose Mourinho said their attitude in the 2-1 defeat at Huddersfield was worse than a friendly match.
Huddersfield took a two-goal lead in the first half, before Marcus Rashford's late consolation goal.
United's first loss of the season leaves them five points behind Premier League leaders Manchester City.
"When you lose a match because of attitude, that is really bad," said former Chelsea boss Mourinho.
United had kept seven clean sheets in their previous eight league games, but mistakes from Juan Mata and Victor Lindelof in the space of five first-half minutes were punished with goals by Aaron Mooy and Laurent Depoitre.
Despite almost 80% possession, United managed just three shots on target.
"I don't even remember a friendly match when our attitude was so poor," said Mourinho. "We could have a second goal and a draw but it would have been undeserved.
"I was a Manchester United supporter, I would be really disappointed because you can accept losing because the opponent had more quality, but not because the opponent had more attitude."