Derby signing Zadkovich likened to Everton's Cahill
Richardson added: "Cahill was at Millwall for a few years and improved all the time. Ruben is a similar sort of player. He is an aggressive, energetic, box-to-box, all-action midfield player.
"He is hungry and determined to do well. I always thought he would have a bright future in the game.
"I gave him a chance and he worked really hard for me. His commitment and work-rate made him very popular with the Notts supporters.
"He works extremely hard to be in the team. When's he's not in the team, he wants to know why. That's probably why he fell out with Gudjon Thordarson.
"I was surprised and shocked when Gudjon Thordarson let Ruben go but I always knew he would bounce back because he is a determined boy. He knows what he wants and where he wants to go.
"Australian players tend to do well in England because they have a great attitude and are very determined."