Louis van Gaal blamed his side's 2-1 defeat at the hands at Danish side Midtjylland on "the law of Murphy".
With United's season already in the doldrums, their best hope of returning to the Champions League next season is now in peril.
United had started positively and went ahead in the 37th minute with a scrappy goal from Memphis Depay but the advantage was cancelled out a minute before halftime when Pione Sisto pounced on a mistake from Michael Carrick and scored with a low shot.
Van Gaal's men were then undone in the 77th minute by a goal from Paul Onuachu. The substitute pinched the ball after a half-hearted challenge from Juan Mata on the edge of the area and swivelled before firing home a low bouncing drive to earn the Danes victory.
David de Gea was injured in the warm-up for United with a knee injury but his replacement Sergio Romero made several fine saves and helped to at least keep the Red Devils in the tie.
"When you start the game and your goalkeeper is number 14 (players out injured) then you are not starting like usual I think," van Gaal said on BT Sport 2.
"A lot of things are happening in the heads of the players and the first 10 or 15 minutes we were not so good. But after that we came in the match, we scored, we could have scored more.
"The second half we didn't win any duel, any second ball. You cannot afford a second goal, but we could have scored also, Jesse Lingard had two remarkable chances, so it is the law of Murphy I think."
When quizzed about De Gea's injury, he added: "We have to wait and see, we have to investigate tomorrow with a scan, we cannot answer now."
Former United midfielder Michael Carrick was more scathing of the team and admitted they were not at the races.
The experienced midfielder said: "It's just not good enough, it's not acceptable, the performance and the result, it's nowhere near good enough. It's got to be a lot better.
"I think we caused a lot of our own problems. We haven't played well enough. We didn't kick on after we scored."