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.
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Moises Caicedo excelled against United once again, two years on from the club's decision to withdraw from discussions to sign the Ecuador international.
When asked if he was happy with the second-half performance, Ten Hag replied: "No, no. First half, we had our better chances and especially in the last part we got overrun in the midfield, it was not necessary but on that point, you have the opportunities by good football and create the chances very well and our level has to be a goal.
"What issue is it? Tactical, mental, but I think we have to do our jobs to the end and when you lose control, you lose the game, you can't lose it.
"I have seen our physical output in the last games and still we go to high levels, we're performing really well, we improved during the season but especially in the last part of the season we are still going up."
United have won one of their nine away games against the teams in the top half of the Premier League and they have taken six points from a possible 33 against the top 12.
At the lower end of the table, United have won away at six of the bottom seven and travel to 15th-placed West Ham on Sunday. "I think the last five (league) games tonight we had four wins, one draw," Ten Hag said. "So I think we were in a good run, we are improving.
"You see first half today, we should have gone up, we create our chances, we still have a way to go, be more clinical and then we have to stay focused until the end of the game."
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