"It's a high ankle sprain. Not exactly sure on the timeline," Shanahan said. "We're going to put him in a boot for a little bit."
On Sunday, Shanahan said Foster could have a broken tibia, but X-rays were negative, per Matt Barrows of the Sacramento Bee.
"It's not that horrible," Foster said, per Casey Pratt of ABC7 in San Francisco. "I'll be back soon. Very, very, very soon."
The 49ers selected the Alabama product with the No. 31 overall pick in the 2017 draft after he tallied 115 total tackles and five sacks in his final season with the Crimson Tide and was named a consensus All-American and the Dick Butkus Award winner. He was tabbed as a critical piece of the defensive rebuild following San Francisco's disastrous 2-14 campaign in 2016.
The 49ers still have NaVorro Bowman and Eli Harold at linebacker, but they will likely need to rely more on Ray-Ray Armstrong and Brock Coyle until Foster is ready to return.