The League Managers Association (LMA) in England had criticised the way in which the Scot's imminent departure was leaked on Monday, the day before he learned the news himself.
But sources at the Old Trafford club insisted the final decision was not taken until late on Monday night, following lengthy discussions with the team's US-based owners, the Glazers.
They claimed executive vice- chairman Ed Woodward had by then phoned Moyes to arrange an urgent meeting on Tuesday at the club's Carrington training ground. It was only then that he was formally sacked.
LMA chief executive Richard Bevan said it was "very disappointed with the nature of David's departure" before he had been spoken to officially.
A United spokesman said: "We do not accept that it was handled in an unprofessional manner."