United came in for criticism from their own supporters for only adding Marouane Fellaini to their first-team squad in a £27.5million deal from Everton.
The champions failed with bids for Barcelona duo Cesc Fabregas and Thiago Alcantara as well as Athletic Bilbao playmaker Ander Herrera.
United also could not get their hands on Everton and England left-back Leighton Baines while a season-long loan of Real Madrid’s Fabio Coentrao fell through.
That perceived failure to add sufficiently to the squad sparked widespread anger among United fans some of whom wrote to the club to complain.
United paid over the odds for Fellaini who has been available for £23.5m – the release-clause figure in his contract until it expired on July 31.
Now the club have replied to fans who have written to them expressing their concerns over new manager David Moyes and his transfer policy.
United’s letters insist Sir Alex Ferguson’s successor has a long-term plan and the full backing of its hierarchy and claim the club exceeded its usual transfer outlay this summer.
They also include a robust defence of Moyes and chief executive Ed Woodward.
The club also said it had achieved its two primary objectives of the window – keeping Wayne Rooney and adding a central midfielder to the squad.