Sport is a results led business which is why so many managers with clubs in the bottom half of the table get the sack. The strength of the squad suggests that Sir Alex got them to over perform last year, there is no doubt that they have underperformed this year and that is down to the manager.
Off the record comments from the players suggests that training was long and tedious in terms of tactics, that obviously didn't work. Sir Alex would have a gameplan, sort that out with the players in training and then in his words 'get them to express themselves' his trophy cupboard shows it worked. Now how many top trophies has Moyes won in football, ah yes none and this season shows why. At the very top the difference between winners and also rans can be a 1 or 2% advantage from team selection, positioning, training, tactics (including successful analysis of your opponents) and performance on the day. The manager is in charge of all of this.
If when you employ somebody and it is obviously not working then you mentor and train the staff, senior staff don't have that luxury as you are paying a high salary, where you expect them to know and prove that their job skills work, so they deliver. If they don't then there comes the point where you have no choice but to show them the door for the sake of the business.
Moyes was given a transfer budget last summer, which he used to try and attract players that weren't interested in joining Man Utd, missed out on those that would have strengthen the squad and then he panicked and bought a player for too much money they didn't need! This failure sowed the seeds for a poor season. Finishing 7th with no European football, especially Champions League football, has cost the club financially at some point you have to cut your losses. Now, I don't think is a bad time as they have time to employ a new manager, so he has the time to spend £100m+ on new players, so they don't have a second poor season.
Moyes is not the only guilty party, other people made mistakes in selecting a man that was not up to the job and the exceptionally high standards expected by the club. Like Chelsea, Liverpool, Man City they may now go through several managers before they find the right one or the senior management may do better this time and get the right one.