Pundits - and with next-to-no coaching experience - like Gary Lineker, Gary Neville and Ian Wright have attacked Amorim since his departure after 14 months in charge at Old Trafford.
But Cunha told DAZN: "I've always been very grateful for everything Ruben did.
"I think it's very easy, when things change, to see the past as only problems, but it's quite the opposite; he was an incredible person. Many new players came because of him, so I think he also plays a big part in the success we are having now."
On claims from pundits that Michael Carrick's tactical change has inspired United's transformation since Amorim's axe, Cunha insists: "The pressure for that tactic to work was so great that we forgot how simple the overall context was and focused too much on the negative.
"But, to be very honest, I think the numbers of football is very misleading. We attack in various ways and end up defending with the formation that everyone says is Ruben's."