Josh, who plays Apple co-founder Steve Wozniak, told US Weekly: "I think it's going to be pretty wonderful. Ashton is going to blow a lot of people away. His performance was absolutely transformational.
"If he looked any more like Steve Jobs, I think it would just confuse a lot of people in thinking Steve Jobs never passed away. It's that uncanny."
The 31-year-old actor, who has appeared in Love And Other Drugs and 21, has been impressed by how much Ashton has thrown himself into the role.
"He really immersed himself. I think he has an appreciation for Steve Jobs, the likes of which I've never seen in any actor who's portrayed a real, living person," Josh continued.
"Ashton is one of the most giving actors I've ever worked with. I was surprised by his work every day," he added.