Louis van Gaal admits Manchester United's shock home defeat to Swansea is likely to "smash" the confidence of his players.
United racked up six straight wins against some of the best teams in Europe during pre-season, but they were given a reality check in Van Gaal's first Premier League game.
Ki Sung-yueng and Gylfi Sigurdsson found the net either side of a Wayne Rooney scissors kick to give the Swans a surprise 2-1 win at Old Trafford.
In typical Van Gaal style, the Dutchman pulled no punches after the final whistle.
The new United manager criticised his players for being nervous. He also said they lacked confidence and did not play as a team.
In fact, the 63-year-old said his first official game in charge of the 20-time English champions was the stuff of nightmares.
"When you have preparation time and you win everything and your first match you lose, it cannot be worse, I believe," Van Gaal said.
"It's a very bad day for us.
"It's very disappointing for the players, the fans, for me, and the staff."
Van Gaal swept into Old Trafford with a swagger after pre-season wins over a sextet of teams with 35 European trophies between them.
But on a chilly afternoon in Manchester, United were brought back down to earth.
Some of United's build-up play was slow, they lacked pace up front, and organisation at the back.
Van Gaal's toughest task of his reign so far will be picking his men up for next week's trip to Sunderland.
"We have built up a lot of confidence and then it shall be smashed down because of this result," Van Gaal said.
"We are not reaching our level. That's the main thing. Because of that we didn't play as a team."
Van Gaal claimed on Friday that Rooney struggled to handle the pressure of being Manchester United captain in Tuesday's friendly win over Valencia.
The United manager thinks many more of his players buckled under the pressure of trying to make a positive start to the season following last year's dire showing under David Moyes.
"I saw a lot of players looking very nervous, making the wrong choices and that is a pity," the former Barcelona coach said.
"These players have to forget that expectation because that is Manchester United.
"You have to cope with that pressure. That is also a process.
"But we have to know that it is only one game of many games.
"You shall not be the champion at this moment. You shall be the champion in May."
With nine players out through injury, Van Gaal handed debuts to Tyler Blackett and Jesse Lingard.
The duo performed reasonably well, although Lingard had his afternoon cut short thanks to a tackle from Ashley Williams.
The 21-year-old left the stadium on crutches and looks set for a spell on the sidelines.
Van Gaal's men got off to the worst possible start when Ki turned home Sigurdsson square pass and the Iceland international drove past David De Gea in the second half after accidentally being teed-up by Wayne Routledge's mis-placed shot.
When asked whether he thought he needed to add new players to his squad, Van Gaal said: "Yes, but I thought that before this match.
"I know in what positions we need better players. But you have to buy only when the player can fulfil the way of playing that I ask."
One plus-point for Van Gaal is that Robin van Persie is close to full fitness following his late return from the World Cup.
"We are now training with Robin for three weeks so the point that he can come in to the selection is closer now," said Van Gaal, who left his compatriot out of his matchday squad.
Garry Monk was delighted to have led Swansea to their first league win at Old Trafford.
The Swans are among the favourites to go down this year, but Monk thinks his team can go on to surprise a few more people.
"Nobody gave us a chance before the game, but we can be better than that," the Swansea manager said.
"We can be sharper and improve in certain areas, but today we were right in many aspects, and we got the right result.
"I'm very pleased overall. We were very good today and showed a great attitude. Hopefully we can have more days like this."