David Moyes was sacked by United this morning.
Purslow told Sky Sports: "David had a number of years working alongside Sir Alex when Manchester United were absolutely dominant in English football - the one big job David was always going to have was when Sir Alex decided to retire.
"For David to announce his departure three months before Sir Alex was a huge mistake - it should never have been allowed to happen.
"It meant that it was July 1 before a new CEO and a new manager had arrived - by then Manchester City had made four top quality signings - and United were always playing catch-up."
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