Jarryd Hayne's deficiencies as a running back have been raised by a San Francisco 49ers coach a day after another coach praised new punt returner Reggie Bush.
49ers offensive co-ordinator Geep Chryst said it was hard for Hayne to learn the important running back task of protecting the quarterback from being sacked.
Chryst said learning pass protection was the hardest thing for rookie running backs like Hayne.
"It's really hard for him to understand all of the subtleties within blitz pick up and all of the protection," Chryst told reporters on Thursday.
Hayne played the first six regular season games as punt returner and back-up running back.
The former rugby league star was dropped from last week's game against the Seattle Seahawks and appears likely to also sit out this Sunday's away clash with the St Louis Rams.
His demotion to the bench came after Hayne fumbled three punt returns and, as a running back, failed to protect Colin Kaepernick from being sacked in a game against the Baltimore Ravens.
On Wednesday special teams coach Thomas McGaughey said he was "very comfortable" with Hayne's replacement as punt returner, veteran Reggie Bush.
"That's the hardest thing for rookies," Chryst, describing the steep pass protection learning curve for rookie running backs, said.
"We've heard that for running backs coming in."
The 49ers' running back stocks have been hit by injuries.
Top running back, Carlos Hyde, has a foot stress fracture but is expected to play the Rams, while second option, Bush, struggled with a calf injury the first six games of the season.
The 49ers are desperate for a victory against the division rival Rams.
The 49ers have just two wins from seven games while the Rams have a three win, three loss record.
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