Despite showing some promising signs, the team's performance was not enough to secure a win in their opening match.
The loss marked an unwanted record for Amorim, who has now lost 15 of his 28 Premier League games in charge, with seven wins and six draws.
This achievement is notable as it is the fastest a manager has reached this number of defeats in the competition, without taking charge of a newly promoted club in that period, since 2009.
Amorim's team will look to bounce back from this loss when they visit Fulham next Sunday, who opened their season with a draw against Brighton.
Arsenal, who won against United, will welcome newly-promoted Leeds to their stadium a day earlier.