His comments, which were made at the Business Insider Leadership Awards dinner, have drawn fire from Amicus. The union described them as "frankly insulting".
As guest speaker at the event, Crozier was asked about challenges that the company faces. Crozier responded that Royal Mail had 15,000 managers, of which "5,000 are fantastic, 5,000 are good and 5,000 are not up to the mark".
He added that it was important for senior management to "take ownership of bad news".
A report in The Daily Telegraph quoted someone who attended Wednesday night's event as saying: "It was a pretty amazing thing for someone to say and completely at odds with what he said about bad news. There were a lot of eyebrows raised over wine glasses."
Amicus's national officer John Hellyer, who represents Post Office staff, defended management.
"Despite suffering 3,000 job cuts in the last year and facing 500 more, Post Office managers have been directly responsible for implementing the Post Office change programme and turning it around from a deficit situation into a major profit earner," he said.
"Adam Crozier has them to thank for this and he will need to rely on their goodwill to introduce other changes. He should consider that before making other ill-judged and frankly insulting comments about his own staff."
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