Michael Carrick said "defeat hurts" after Manchester United's 2-1 loss at Newcastle on Wednesday – the first time he has been beaten as manager of the club.
William Osula came off the bench to score the winner for the 10-man home team at St James' Park, putting a dent in United's push for Champions League qualification.
Former United midfielder Carrick, who took charge at Old Trafford in January, had won six and drawn one of his seven games in charge before Wednesday's trip to Newcastle.
He was also unbeaten during a previous interim stint at the helm, in 2021.
Jacob Ramsey's dismissal for a second bookable offence sparked a frenzied conclusion to the first half, during which Anthony Gordon put the home side ahead from the penalty spot before Casemiro restored parity.
But United failed to capitalise on their man advantage in the second period and Osula produced a moment of magic at the death, sprinting down the right before cutting inside and curling an unstoppable shot past Senne Lammens.
"We are not happy the way we played tonight," Carrick told TNT Sports.
"The way the game panned out we had it in our hands largely but credit to Newcastle and the way they approached it.
"We knew it would be tough but we navigated the game to a position where we could kick on but we didn't. Bitterly disappointed really.
"I don't think it was the 10 men, we just didn't play good enough. We can't make excuses for that. We all take responsibility for that."
Carrick said his team, in third place in the Premier League, "need to get back to work and be better for the next game" as they seek to finish in the top five.
"We've lost one game, we haven't played well enough but in the grand scheme of things we are in a decent position," he said. "Tonight hurts but we will be better for the next one.
"You have to get back at it, work hard and do the things we did to win the games we've won. We can do that, we are a good team. Tonight wasn't for us."