Manager Erik ten Hag stressed the importance of Manchester United maintaining focus until the final whistle and preventing late goals after Brighton & Hove Albion secured a 2-1 Premier League victory with a goal in added time.
United conceded two soft goals at the Amex where Brighton striker Danny Welbeck scored against his former club before an unmarked Joao Pedro scored the winner in the fifth minute of stoppage time.
Once a team renowned for scoring late goals in the Premier League, United have now lost six times since the 2022-23 season thanks to goals scored in or after the 90th minute -- more than any other side.
"We have good potential but we have to keep going until the end. We concede a goal in stoppage time," Ten Hag told the BBC.
"The game is finished when the referee whistles three times. That is what we did not do well, so we have to improve that game management.
"Two soft goals and we have to be more clinical in both boxes and act better as a team. It's a pity that we are without any points here. We have to get up and go for the next big game (at home to Liverpool)."
At 1-1, United had a good spell and nearly made it 2-1 when Alejandro Garnacho's shot on target was deflected in by Joshua Zirkzee, with VAR chalking off the goal because the Dutch forward was offside while sliding in.
Had Zirkzee not made contact, United could have completed the comeback but Ten Hag refused to blame the 23-year-old who scored on his debut in a 1-0 win over Fulham last weekend.
"It was difficult. In the moment he was on the slide. If he had read the situation better, maybe he could have avoided it but you can do nothing once you are on the slide, you can't avoid the ball," Ten Hag said.
"It's tough but also we created many more chances to score a second goal. We showed resilience and fought back, we had our opportunities to score more goals. It's disappointing when you don't win and then you lose."