Louis van Gaal has revealed that Radamel Falcao is facing a fight to feature for the club because his third tactical "remodelling" of the side creates room for only one striker.
Falcao has scored only once for United since the club agreed a £6m signing-on fee and wages of around £265,000 a week and there are concerns about his fitness with a calf injury, which has kept him out of the side for the past three weeks.
And, even when the striker regains fitness, Van Gaal's need for better "balance" in the side, which has been lacking defensive ballast, means Falcao faces a fight for the solitary striker's position, with Robin van Persie ahead of him in the ranks.
"I have to decide which striker," Van Gaal said. "It is not his fault. It is not his fault [that] with the other system I could play with two strikers but then the balance wasn't good.
"We had difficulties in the last half an hour [of a game.] That is why I have remodelled the structure of my team for the third time.
"Since then we have less goals in our favour but also less goals conceded and we have played against the better teams in the Premier League in that time on an equal level, more or less."
Van Gaal has switched from 3-4-1-2, to a diamond midfield and, against Manchester City, a 4-4-1-1 - a formation he seems to have decided to continue with.
The United manager was unable to state when Falcao would be back in the side and said that the defender Marcos Rojo's dislocated shoulder injury was of the type that could occur at any time.
United have decided against an operation, which could mean the player is back within fewer than six weeks, but Van Gaal was uncertain. "The human body has been made by God, not by me," he said.
Van Gaal admitted that United's disappointing start to the season has left him feeling "lousy".
United currently lie in 10th place in the Premier League with 13 points after 10 matches following last Sunday's derby defeat to Manchester City at the Etihad Stadium.
David Moyes, who was sacked in April, had 17 points following the same number of games in his brief tenure. It is also United's worst start to a league campaign since the 1986-87 season, pre-dating Alex Ferguson's tenure.
"It's not good enough. I feel myself very lousy and for the fans also," Van Gaal said.
"Also the board, because they have a great belief in me and my staff.
"When you have 13 points after 10 matches you are not doing well.
Van Gaal acknowledged that the growing injury list at Old Trafford has hampered the implementation of his football philosophy.
"When I want to select the same line-up, I cannot," he added.
"You cannot create consistency and that's important in football. But I don't want to complain or cry, because I have great belief in not only the players but the staff and the players and also the supporters who are supporting me."
Van Gaal insists there will be no quick fix at United.
In fact, the former Holland coach says it may even take three years to rebuild a club whose name is synonymous with success.
''We are in a process and that process shall take more than one year,'' he said.
''It shall take three years.
"I cannot say when the results are coming. When I did it with Bayern Munich it was December 8. It can be February 1. But it doesn't matter.
"This is a process of three years. I have signed for three years.
"It takes a lot of time. But at the end, when they know it and they are doing the things with conscience, the results shall come.
"Our goal (this season) is to get back in the Champions League."
Van Gaal struck the table in front of him on one occasion during a tense press conference at the club's training ground in Carrington.
His mood has been soured by the news that he will be without any fit senior centre-halves for today's game against Crystal Palace.
When asked whether he would enter the transfer market for a defender in January, Van Gaal, who has been linked with Aston Villa's centre-back Ron Vlaar, said: ''It is clear that a lot of things happen and you can judge better in good times than you can in bad times.
''Maybe you are right (about buying in January) but we have to see that.''
Van Gaal was giving little away about the potential make up of his back line for today's game, but it is likely that Michael Carrick will have to play at centre-back alongside teenager Paddy McNair or the equally inexperienced Tyler Blackett.