Louis van Gaal accepted responsibility after Manchester United crashed to a 2-1 loss at home to Swansea City on Saturday in his first Premier League game as manager.
The 63-year-old Dutchman is bidding to transform United's fortunes after a dismal seventh-place finish last season, but his tenure got off to a similarly disappointing start as Swansea claimed a first league win at Old Trafford. (Aaron Ramsey gives Arsenal a late win)
Ki Sung-yueng put Swansea ahead in the 28th minute and although new United captain Wayne Rooney hooked home a 53rd-minute equaliser, Gylfi Sigurdsson gave the Welsh side victory with 18 minutes to play.
"We were very nervous in the first half, made the wrong choices, and that is a pity. In the second half we never played as a team. Therefore I'm responsible," said Van Gaal.
"It is very disappointing that we have lost our first home match. We didn't reach the level we can play at.
"That is disappointing because we have done it much better and when you cannot do it in your first home match that is disappointing, especially for the fans."
Van Gaal set his team out in the 3-4-1-2 formation that he had successfully introduced in pre-season, but Ki's goal forced him into a half-time switch.
Nani came on for striker Javier Hernandez, with Ashley Young moving to left-back, but although Rooney equalised soon after and then hit the post from a free-kick, United struggled to create further chances.
"We had more chances than Swansea, but that shall say nothing about the result," Van Gaal added.
"It is the goals that count. I thought we would win (at 1-1), but because we are not playing as a team in the second half, we made the wrong decisions going forward.
"My players did their utmost best, which I said in the dressing room, but you also need to play as a team and reach your level."
With new left-back Luke Shaw sidelined by a hamstring injury, United had only one close-season signing in their starting XI in the shape of former Athletic Bilbao midfielder Ander Herrera.
Injuries obliged Van Gaal to hand competitive debuts to youngsters Tyler Blackett and Jesse Lingard, but he dismissed suggestions that the result would force him to take emergency action in the transfer market.
"No, because that is a short-term assault. Of course we need defenders, but I have already said that in former press conferences," he told BT Sport.
"You cannot say because of one game that we lost, and all the others we won, that it is the defence (that is responsible). It's the team."