The 49ers also placed WR Jauan Jennings on the practice squad injured list.
Mostert missed some time earlier in the season with an MCL sprain.
Mostert, 28, wound up signing on with the Eagles as an undrafted free agent out of Purdue back in 2015. He had brief stints with the Dolphins, Ravens, Browns, Jets and Bears before signing on to the 49ers’ practice squad in 2016.
The 49ers brought Mostert back on a one-year exclusive rights contract in 2018 before tendering him a restricted offer in 2019. He later signed a three-year, $8.7 million extension with San Francisco.
However, the two parties agreed to a restructured contract this summer.
In 2020, Mostert has appeared in four games for the 49ers and rushed for 303 yards on 51 carries (5.9 YPC) to go along with 11 receptions for 150 yards and two touchdowns.