This is a potentially interesting development. San Francisco 49ers offensive coordinator Geep Chryst addressed the media on Thursday, and he said Jarryd Hayne would remain the backup running back if Reggie Bush is unable to go on Sunday. He did not specifically address Mike Davis's role, although I would imagine Davis would be active since the team would want a third running back available.
The fact that Chryst said Hayne would be the backup does not necessarily count as a quick strike against Davis. It is possible the team is just more happy with Hayne than Davis, and that's that. However, it is also possible this is just a feint of sorts to keep the Steelers in the dark. I do not expect Bush to play on Sunday, which means we will get an answer pretty quickly on this.
Hayne got off to a slow start in his NFL debut. He fumbled the punt return, and followed that up with a 1-yard run. However, he eventually had an 11-yard punt return and a 9-yard rush mixed in. He finished the game with four rushes for 13 yards, one reception for seven yards, and the 11-yard punt return. Not exactly a big night for him, but certainly a first effort. And while he is an older rookie, he is a rookie nonetheless. More importantly, he's a rookie who has not played organized American football before this year. He is making progress, and the next step will be for him to show further improvement week-over-week.