Since Michael Carrick has replaced Amorim at United, he has won his opening two games in charge against Manchester City and Arsenal.
Speaking on The Rest is Football podcast, he said: "What on earth is Ruben Amorim sat doing, thinking, looking at those two performances? I mean honestly, I don't know what he'll be thinking.
"It's absolutely crazy (to not try and change formation). The arrogance to say, 'No, I'm not doing any other thing. This is my way or the highway.'
"I mean, to not even shift or to accept that this is probably not working, 'Maybe I should try something else.'
"He put all his eggs into that basket, and if that didn't work, he was done. And it didn't."
On Carrick, Shearer said of his fellow Geordie: "He's made an unbelievable start, maybe even surprised himself. I don't know, but I think ask me that in two months time (about whether Carrick should be handed the job long-term)."