New Manchester United manager Louis van Gaal said his players' confidence would be "smashed" after the worst possible start to his reign at Old Trafford.
United, who won six friendlies under Van Gaal in pre-season, began their Premier League campaign with a shock 2-1 home defeat by Swansea.
Van Gaal said: "When you have preparation, win everything, then lose your first match, it cannot be worse.
"We have built up a lot of confidence. It will be smashed because of this."
|Van Gaal's pre-season record|
|WIN 7-0 v LA Galaxy|
|WIN 3-2 AS Roma|
|DRAW 0-0 Inter Milan (United win 5-3 on penalties)|
|WIN 3-1 Real Madrid|
|WIN 3-1 Liverpool|
|WIN 2-1 Valencia|
Swansea's victory was their first in their history in the league at Old Trafford, and saw United lose at home on the opening day of the season for the first time in 42 years, since Ipswich beat them 3-2 in 1972.
It was a defeat and performance reminiscent of United's struggles at home last season, when they lost seven league matches under David Moyes and were beaten by the Swans in the FA Cup.
On top of United's failure to strengthen their squad significantly since Van Gaal's arrival in July, their list of injured or unfit players meant he gave debuts to two academy graduates - Tyler Blackett and Jesse Lingard.
|Former Netherlands international Ruud Gullit on BBC Match of the Day|
|"Van Gaal has a big job. The Premier League is different. You can see during the game the United players were thinking about where they had to be. When he played 3-5-2 for the Netherlands it was necessity because we had bad defenders. If you were a Swansea player would you have been afraid of that line-up?"|
With Luke Shaw injured, United could field only one summer signing, £29m midfielder Ander Herrera.
But Van Gaal swatted away suggestions that the display and defeat against the Welsh side will make him step up efforts to strengthen his side before the transfer window shuts on 2 September.
"Yes, I need new faces but I thought that before this match anyway," he said.
"When we were playing in the United States I thought the same. I know the positions where we need better players."
|Gone till November?|
|New Manchester United boss Louis van Gaal warned in July that it normally takes three months of indifferent results before his methods bear fruit at a club.|
|"Every club where I have been, I've struggled for the first three months," he said. "After that, the players know what I want. I am very direct. I say things as they are so you have to adapt to that way of coaching. It's not so easy."|
He also dismissed questions that the defeat has made him realise the scale of his task as he looks to take a team that finished seventh last season back into the top four and challenge for the title.
"I have said before that I know how big a job I face here," Van Gaal explained.
"Our wins in the United States and against Valencia meant that the expectation was so high.
"In the first half I saw a lot of my players looking very nervous and making the wrong choices. The players have to get used to that expectation because this is Manchester United and they have to cope with that pressure.
"Of course I am very disappointed to lose - for the players and fans and for myself and my staff.
"But we have to remember this is only one of many games and nobody is champions now. You are the champions in May."