The Red Devils lost 2-1 at Old Trafford on Saturday and despite stating throughout pre-season that he wants the club to challenge for first place, he has admitted that it is not possible with his current squad.
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"But also with other players it's also a question of if they can play like a team and if they can reach the level. And today we don't reach that level and then you cannot play as a team so easily.
"And that we have seen today. But in spite of that we could have won also. But when you don't play like a team and you miss the chances then it's very difficult because every team should also create possibilities or chances against us. Because we are playing with a big space behind us. That's always difficult to defend."
Van Gaal fielded youngsters Tyler Blackett and Jesse Lingard in a 3-5-2 formation at Old Trafford after losing a raft of experienced players on free transfers and new signing Luke Shaw to injury.
And he says he already knew of the importance of signing new players before Saturday's defeat.
"Yes, but I have thought that before this match also so that is not the difference," he added. "But when we are playing in the USA, I thought the same. I know in what positions we need a better player. But you have to buy only when the player can fulfil the way of playing what I ask. And we have to wait and see. But we could have won today also."
On the subject of showing players the Old Trafford exit door, Van Gaal added: "I don’t think it’s only up to me, it’s up to the players to decide because we have given them problems. I shall tell the players what I think and what I want from them but I shall never say they have to go."
United fell behind to a Ki Sung Yeung goal in the first half, but even after Wayne Rooney equalised with an acrobatic effort from close range it was Gylfi Sigurdsson who struck the decisive goal.
And Van Gaal admitted he made the decision to ditch his much-heralded 3-5-2 system during the first half.
"[I decided to change] in the first half. They made the pitch very big and we had to press in front, so 4-3-3 is better system to press. In the beginning we did well, we scored, and we had chances to make it 2-1 but chances are not enough, you have to [score] the chances.
"I think that we can do it with both systems. You have seen the first half that we are not playing very good and we are behind and that's why I have changed the system and this system they have played always in Manchester United.
"And then I changed it again. So I do everything to win. And to my players I cannot say that you didn't do your utmost best. Unbelievable work and fantastic mentality they have shown. But reaching the level is different, not only running but also using your brain and then you can play like a team. But that we have failed today."