Ole Gunnar Solskjaer admitted his Manchester United side lacked ideas to prevent a shock loss by Sheffield United, but was left furious that the Blades' opening goal was allowed to stand.
The Premier League's rock bottom side stunned Old Trafford when they took the lead in the 23rd minute through a Kean Bryan header before later going on to win 2-1 after Oliver Burke struck a second-half winner following a Harry Maguire equaliser.
Shortly after the opening goal, United thought they had levelled the game when Anthony Martial rolled the ball home, only for a foul to be given after Harry Maguire was adjudged to have impeded Blades stopper Aaron Ramsdale.