The San Francisco 49ers placed Ahkello Witherspoon on injured reserve, officially ending his second NFL season. The cornerback sustained a sprained posterior cruciate ligament (PCL) during the first quarter of Sunday's victory over the Seattle Seahawks.

The 49ers signed defensive back Tyvis Powell to a one-year deal to take his roster spot.

Witherspoon landed awkwardly out of bounds while attempting to tackle Seahawks running back Chris Carson. He was assisted to the locker room and did not return to the game. Witherspoon's recovery is expected to last a few weeks, which means it should not impact his availability for 2019 or the team's offseason activities.

"I wish he could have finished these last two games," Shanahan told reporters on Monday. "I think he definitely was finishing better than he started, but his story's not written yet. He's got to improve, and hopefully, he'll come back here ready to go next year, and take off the way he finished."

Rookie cornerback Tarvarius Moore, who replaced Witherspoon on Sunday, will start during the 49ers' final two games of the season.

The 49ers made Witherspoon a third-round draft pick out of Colorado in 2017. He has appeared in 26 games with 21 starts over his two seasons and registered 69 tackles, 11 passes defensed, two interceptions, and a forced fumble.

Powell appeared in four games with one start for San Francisco this season. He registered five tackles and one forced fumble on defense in addition to one special teams tackle before being waived by the team on November 12. Powell was signed to the New York Jets practice squad on December 4 and released on December 11.

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