Erik ten Hag conceded Manchester United got overrun in midfield in the second half against Brighton.
Alexis Mac Allister's 99th-minute penalty consigned United to their fourth defeat at the Amex Stadium in the last five years and they are yet to win at Brighton's stadium with a crowd present.
Ten Hag sent on Marcel Sabitzer and Jadon Sancho for Fred and Antony midway through the second half but Christian Eriksen was unused.