Ole Gunnar Solskjaer has blamed the finishing and defensive errors as they lost 2-1 to Crystal Palace.
Palace led for much of the game through Jordan Ayew’s first half strike before Daniel James’ second goal of the season in the 89th brought it level.
Patrick Van Aanholt’s injury time goal won the game for Palace as the ball squeezed past David De Gea at the near post.
Marcus Rashford also missed a second half penalty as United’s woes from the spot continue.
Solskjaer told BBC’s Match of the Day: “They scored 2 and we scored only one. We just were not clinical enough in front of goal and they scored two very easy goals. I thought we got our act together second half, we just couldn’t hit the target. We have dominated the game but we have not controlled it.
“We should have dealt with the last five minutes better. We defended poorly at times today and it cost us. Sometimes you do not find that end product and that happened today. We did not test their keeper enough. It cost us dearly today.”
“Two games, two missed penalties. It’s part and parcel of it. Sometimes it happens. I have missed a few myself. If we score them it’s a different story. If we keep putting ourselves in those positions we will get more penalties and we will score. It is just one of those things. We trust them to score again”