In fact, he doesn't even think it will be a question.
"I'm sure he'll make it, I don't think there's any doubt in anybody's mind he'll make the roster," Bush told The Daily Telegraph of Sydney, via CSN Bay Area. "He has all the talents and athleticism to be able to play in this league and to be able to play at a high level in this league. I think making the team is actually the least of his worries."
Hayne is a 6-2 running back with a natural ability to catch the ball in traffic. He's had a longstanding personal endorsement from Bush, but the true test will be this summer once the team opens up organized activities.
He is not the first rugby star of any variety to give the league a shot, but if he actually does make the roster, he'd be one of the most successful.
Most recently, Rugby union star Hayden Smith latched on with the Jets in an attempt to become a tight end. His run ended after five games and one reception.
But something about Hayne's style of play and conversion have people expecting more. The Niners have not ruled out trying him at other positions beside running back, though coach Jim Tomsula is already allowing his mind to wander.
"He's a guy used to being downfield catching a ball with people running at him," Tomsula said. "He's used to making tackles in open spaces. You've got a guy used to carrying the ball, catching the ball and avoiding. Obviously, with the explosive qualities that he has, you can't help but be excited."
Maybe Bush is right after all.