Purslow was in charge at Liverpool when they dropped out of the Champions League.
"On Sunday (after United's 2-0 defeat to Everton) it became mathematically impossible for them to qualify for next season's Champions League - and the message coming out today from the Glazers was that was the minimum requirement," Purslow told Sky Sports News.
"Last season they won the league by 11 points - in their worst nightmares they could not have expected to find themselves out of this tournament.
"They've been first or second in 18 of the 21 years of the Premier League and never lower than third - I'm quite sure they could not have anticipated this level of under-performance.
"From a business perspective - and that's what this is all about - the club is structured in every respect to be playing on a Tuesday or Wednesday evening at this time of year against Real Madrid, Barca and Bayern Munich.
"Their sponsorship deals expect that, their players have contracts and wages that reflect that level and when that's not happening it's very damaging.
"This is a company that floated on the New York Stock Exchange less than two years ago - today it's worth nearly three billion dollars - do you think any of those investors would invest in a club outside of the Champions League? I think not.
"If I was chairman of a three billion dollar company - looking at what is required this summer, I would have made a change."
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