Hackworth was delighted with the way the World Cup winner performed in his short stint and knows he will get even better for the Union once he adapts and finds fitness.
"Kleberson is one of the best professionals I've been around in the short time that I've been around him," Hackworth said.
"He understands that you come to work every single day, you do everything you can as a player to put yourself in a position to be selected. He said, 'Whenever you want to put me in, I'm ready.' And that's what you want from a pro."