The Premier League champions have released a response to critics, reiterating their full faith in David Moyes and stating that keeping Wayne Rooney was the top priority
Marouane Fellaini was the only high-profile arrival at Old Trafford in the summer, while public pursuits of Thiago Alcantara, Cesc Fabregas, Leighton Baines and Ander Herrera all failed to result in deals.
But United insist they met their key targets in the summer transfer window, and spent more on strengthening the squad than in previous years.
"The club had two main priorities for the transfer window," the letter reads. "The first was to keep Wayne Rooney. He is a world-class player and the manager is very keen to have Wayne as a key part of the club's drive to retain the Premier League trophy. That aim was fulfilled.
"The second priority was to add a midfield player, and the capture of Fellaini has met that need.
"Add to that the fact that Wilfried Zaha is a new signing and that the manager has added the highly-rated young player Adnan Janazaj to the squad and the team does in fact have a different feel to it than the one that finished last season.
"The club has invested over £40 million acquiring new players this summer, as well as re-signing Nani on a long term deal - more than it has traditionally spent on transfers."
United admitted they had failed to reach agreements with other clubs for some of their transfer targets, but insist they will continue to back manager David Moyes in his attempts to strengthen the squad.
"Additional resources were there to add to that but it was not possible to agree with other clubs for the right players to leave. The key point there is that the players we buy have to be the right ones for Manchester United; not just any player.
"The club has always backed the manager in the transfer market, as Sir Alex has said on many occasions and it will continue to do so. The club has demonstrated its belief in, and commitment to, David Moyes through the award of a six year contract.
"Allowing him to shape the future Manchester United team is a long term project not an eight-week panic. He must be given time to assess his new squad and come to his own decisions on which players he feels will strengthen it."