SANTA CLARA, Calif. _ Linebacker Reuben Foster's availability is in doubt for the 49ers' upcoming visit to Seattle.
Foster sustained a high-ankle sprain and will "miss time," the NFL Network reported Monday.
"I told him it's a long season, so he's just got to get that thing right," defensive tackle DeForest Buckner said after Sunday's 23-3, season-opening loss to the Carolina Panthers.
A further update is expected later Monday when coach Kyle Shanahan addresses the media.
"Trust me, if you can't put pressure on it, you should be worried, but if you can put a little pressure on it, you've got some hope," Foster said Sunday. "That's all I'm hoping for."
Last season, the 49ers lost outside linebacker Aaron Lynch to a high ankle sprain for five games. After Sunday's first road trip of the season to Seattle, the 49ers return home on a short week to host the Los Angeles Rams on Thursday night, Sept. 21. Three consecutive away games are next, to Arizona, Indianapolis and Washington.
Foster's right ankle buckled beneath him with 3:10 remaining in the first quarter. Carted to the locker room, Foster underwent X-rays that showed no fracture and he watched the game's remainder from the 49ers sideline.
"He couldn't be out there with his brothers so he wanted to support. I respect him for that," Buckner added.
Safety Jacquiski Tartt said: "Reub's a big part of the defense and to see him go down, it gets to you, but at the same time, we stayed together."
Foster overcame low-ankle sprains in training camp to secure the starting spot at weak-side linebacker. After Sunday's debut, he said it was "sad I had a nick in the ankle."
Ray-Ray Armstrong replaced Foster and figures to do so until the first-round draft pick is medically cleared to return.
