SANTA CLARA — The 49ers could be without promising rookie linebacker Reuben Foster for multiple games after he suffered a high ankle sprain in his debut against the Panthers, coach Kyle Shanahan said.
"We'll put him in a boot here for a little bit," said Shanahan. "Those tend to be a month, a little more. I can't exactly put a date on it, but that's my experience with high ankle sprains."
Foster had to be carted off the field following his 11th snap as a pro when his right ankle twisted underneath him as he tried to tackle fellow rookie Christian McCaffrey late in the first quarter. Foster, to that point, was sparking San Francisco's defense with three tackles, including one for a loss, and a near interception of Cam Newton.
"He was playing well. He was around the ball a ton," Shanahan said.
Foster underwent X-rays that came back negative. He walked back to San Francisco's sideline and tried to make his case to the training staff to return, but was held out.
The 2016 Butkus Award Winner was the story of 49ers training camp, ascending to the starting lineup after veteran free agent addition Malcolm Smith went down with a season-ending pectoral injury.
San Francisco traded back into the first round of the draft to select Foster with the 31st-overall pick after he was widely considered a top-10 talent. General manager John Lynch said Foster was the third-ranked player on the team's draft board.
The 49ers next travel to play the Seahawks, where they'll be tested by elusive quarterback Russell Wilson.
Veteran Ray-Ray Armstrong is expected to replace Foster after filling in Sunday following the injury.
"I thought Ray-Ray did a solid job," Shanahan said. "He stepped up and came in, played real hard, didn't lack any effort. He was around the ball and made some plays."
Armstrong, 26, played 45 snaps and was credited with six tackles in Foster's absence. He was flagged for pass interference deep in San Francisco's territory before the Panthers scored their second touchdown.
Armstrong in 2016 earned a starting job next to NaVorro Bowman but was lost for the season in Week 2 when he suffered a torn pectoral. The former college safety is known for his athleticism and coverage skills.
The 49ers lost 23-3 Sunday but only allowed 287 yards of offense to the Panthers.